Nikon 24-70mm f/2.8 off by 1 stop

simba

I recently purchased Sigma 35mm 1.4 and Nikon 50mm 1.4, and compared them to my Nikon 24-70mm 2.8 with the corresponding focal length. I set ISO to 100 and used aperture priority. I compared the shutter speed and depth of field of the images. To be my surprise, the shutter speed and depth of field of the 24-70 was exactly off by 1 stop. That is, the shutter speed and depth of field of the image taken by 24-70 with aperture 2.8 was the same as those taken by Sigma 35mm with aperture 4, etc. The same result also happened to 50mm. It looks like Nikon is cheating on the zoom lens. Have anyone experienced the same problem?

PB_PM

It's not cheating, it's called the science of light transmission. A zoom generally has more glass, thus lower light transmission. This is no secret or surprise. Any more conspiracy theories?

Golf007sd

Your are not getting cheated, nor is Nikon doing it. It come down to the science of how the lens function. A 2.8 telephoto lens handles light very much differently than a prime lens that has an aperture of 1.4. It would be do you some good if you study on how these optical lenses work before you state that you are getting cheated.

simba

It is understandable that a zoom lens takes longer shutter speed due to its efficiency. Could you provide some explanation why depth of field is different? It is good to know how to offset the results of a depth of field calculator when lenses are being considered.

Golf007sd

@simba: Please do a search on our forum regarding DOF and you will find many topics covering it. Moreover, you will find many of the conversation cover DX vs FX, F-Stop, distance to subject, lighting and lenses that come in to play as it relates to DOF.

John

Ok, let's get few concepts straight before we begin.

The aperture (a.k.a. f-stop) is relation between the opening of the diaphragm and the focal length of the lens. So, for a given focal length the diaphragm opening will be the same on different lenses as long as you use the same aperture. This means that 24mm f2.8 will be the same regardless of lens.

Aperture also determines depth of field. For a given focal length and a given distance to the subject the depth of field will be the same when you use the same aperture.
This means that when you shoot a target at 1m distance and at f2.8 and use 2 different 24mm lenses to do this you will still have the same depth of field.

All this has nothing to do with light transmission properties of the lens.
Even high quality optical “glass” will absorb some light. This amount might be very small but there will be some absorption. Different lenses might have different light transmission properties.
For example: A very short 50mm prime lens has very little glass in it so it will transmit a lot of the light that entered the lens. A thick 24-70mm zoom needs a lot of glass to be able to zoom while maintaining image quality (sharpness, limited distortion, limited chromatic aberration,…).
More glass means more light absorption so less light reaching the sensor for a given focal length/f-stop/shutter speed combination.

Now that we have this technical background out of the way we can investigate your claims.

1) The Nikkor 24-70 f2.8 absorbs 1 full stop of light compared to the 2 listed prime lenses.
It’s possible but I do doubt the stated amount. The zoom lens will indeed absorb more light as explained above but I find it hard to believe that this would be a full stop of light. I know people who shoot the 24-70 in studio and use an external light meter and I’ve never heard them complain about underexposure.
If the loss of light would indeed be this high the resulting images would all be underexposed as the external light meter does not know that it has to compensate for the additional light absorption from the glass.
So, I would like to see your images (including exif data) so we can look at the problem ourselves.
The best way to demonstrate the problem would be to:
- Turn of all the automatic iso, HDR, active D, Active D lighting, noise reduction, long exposure noise reduction,… functions.
- Shoot an evenly lit sheet of white paper (only the white paper in image, full frontal shot so not at an angle). You can put a small black dot on the piece of paper to allow the autofocus to focus.
Do this with one of your prime lenses. Set aperture and shutter speed manually according to the exposure that the camera tells you.
- Now create the EXACT same image (shutter speed, aperture, focal length, focus, iso,…) with the 24-70.
- Output them both as picture controle “standard” .jpg files straight from the camera.
- Look at the histogram in photoshop and see is they are actually different. If they are (which they should be at the stated 1 stop difference then you should post you pictures online so we can look at the issue and see if it’s indeed due to light transmission or if other issues are in play).


2) The nikkon 24-70 at f2.8 has the same depth of field then the sigma 35mm at f4.
Assuming that both these lenses are FX format then this should not be possible as there is a direct correlation between focal length, distance to subject, aperture setting and the resulting depth of field.
So, show us 2 images (with exif data).
One of the Nikon 24-70 at 35 mm and f2.8 and the other of the sigma 35mm at f2.8.
Use the same distance to subject (I would suggest using a rules as the subject so we can see where the in focus area starts and ends) and make sure that all settings (aperture, shutter speed,…) are the same.
Then show us the results because issue 2 should be impossible. Either you made a mistake (happens to the best of us) or there is an actual issue.

I ‘m definitely looking forward to seeing the results.

One final word of advice. Use a tripod for all the test shots, focus using the central focus sensor (or use live view) and provide the resulting images in the highest resolution the camera can output (but shoot jpg so you don’t have raw convertor issues messing with your final result)

haroldp

It is possible that the location / distance of the actual aperture from the focal plane might affect DOF, and if on the 24-70 f2.8 which is a physically long lens for it's optical Focal length, that the aperture though correctly sized for f2.8 is further out and therefore would have a tighter transmission cone and more DOF.

While the t-stop of a complex zoom is less than the f-stop, and less than the t-stop of a simpler prime, I agree that a full stop is too big a difference and would question the test.

We should also keep in mind that a 35mm f1.4 for an FX SLR is not a simple lens and is both tele-centric and has a complex optical design with lots of elements.

The 24-70 f2.8 is a staple lens for me and I have never had reason to question its performance.

Regards .... H

Correlli

I am with John on this one. At a given focal length and f-stop the depth of field should be about the same (some minor variations due to the optical quality of each lens might influence this slightly).

About 20 years ago I did a test at university comparing a Nikkor 50 mm f1.8 agains some 2.8 Tamron zoom (both manual focus lenses) and the light transmission difference really was about one f-stop. But this was before Nano-Coating and all this.

So I would also suggest to repeat the test.

@haroldp: the Sigma is not telecentric. Telecentric lenses are very specialized lenses for machine vision and measuring applications. The ones I worked with are object side telecentric. This means that the object does not change its size in the image within a certain range. These lenses usually have a fixed working distance and the diameter of the front element must be at least the size of the object. So I think one of us misunderstood something about either the lens or the meaning of tele centric.

JJ_SO

@Correlli the f/stop is not a dimension for light transmission. It's the relation of aperture to focal length and therefore the writing style f/2, i.e.

It doesn't say anything to the amount of light passing through the aperture. If you use a grey-filter, the aperture doesn't change, either. Lots of glass inside a lens is nothing else than a sort of grey filter.

spraynpray

I am not in agreement with some of what has been posted. A 24-70mm f2.8 zoom set at 35mm and used wide open should be putting the same amount of light on the sensor as a 35mm f1.4 is when set to f2.8. The difference in the length of the lens is compensated for by the diameter of the glass.

For instance - when was the last time you saw an f1.4 lens of a given focal length with a smaller diameter than an f4 of the same focal length?

Assuming simba is not doing something wrong, then something isn't right with one of the lenses.

I have been wrong before and I just might again sometime, but what I have written above seems totally logical to me... :P

JJ_SO

@spraynpray, no it's not. Aperture is a relation of the diameter of a given aperture to the focal length. Different types of glass with different diffraction indexes are not part of that relation, but do change the necessary shutter time.

Msmoto

I am of the belief the entire topic is a result of an error in the procedures of measuring this. Unless the results of any scientific test are replicated by another investigator the initial conclusions have little merit.

spraynpray

@msmoto: Agreed.

@JJ: I don't get it (obviously) - but then I would like Jonnyapple to post on this as he is the only person on this forum that I know of who is qualified to post on this topic.

People seem to be saying that f2.8 at any given ISO does not mean the same shutter speed. Really? First time I EVER heard that. Wow.

JJ_SO

Don't give me a cheap reason to test B-)

But matter of fact, it is. And we usually don't realize it because of our TTL metering out the whole loss of light because more or less glass elements.

But you are familiar to the fact, that a lot of glass screens in a row give you another picture than only one screen when photographing through it? The aperture doesn't equalize that.

spraynpray

Blimey. I thought that the amount of photons coming though to the sensor is the same on any lens at the same aperture and so the diameter of the glass was altered to ensure that (what I mean by that is that f2.8 on that 24-70 zoom would be the same diameter to keep the DoF right but to achieve the same exposure the diameter of the lens would have to be larger to gather the same amount of light). Actually, what I just said still makes perfect sense to me!

My rifle scopes vary greatly in their low light performance just by their object lens diameter - the tubes are all 25mm.

shawnino

Let's reiterate what we're actually talking about here in the first case: transmissivity.

Aperture area is an exact formula: The area of light captured = (pi)(f/2N)(f/2N)
where pi = 3.1415..., f is the focal length and N is the f-number.
I've written it that way because I can't find out how to cram in a formula to the message, but it's essentially the area formula of a circle, "pi-r-squared" with (f/2N) acting as radius here. That's just geometry's formula, no more and no less.

Transmissivity can start with the aperture area, but accounts for lost light absorbed/reflected by glass. Zoom lens designs are more complex than prime lens designs; everything else being equal, a zoom lens should transmit a bit less light than a prime.

The claim is that the zoom lens is transmitting a stop less light than the primes. A full stop is an awful lot. I'm not claiming it's impossible but on a pro-grade lens I would think it's unlikely. I would very much like to see the test shots.

As to DOF, under standardized conditions (distance to subject, sensor size...) this is back to geometry. Strictly quantitative, not qualitative. Long read, but here for those interested:
http://en.wikipedia.org/wiki/Depth_of_field#Derivation_of_the_DOF_formulas

If OP is getting markedly different DOF under standardized testing, something truly is amiss--world's weirdest lens(es), or a camera body reading/displaying data incorrectly, or... ???

I would want to see the test shots.

spraynpray

@Shawnino: Is your last post aimed at anybody in particular? It doesn't seem to directly address my last post, (which still makes sense to me BTW).

If the 'transmissivity' of any stack of glass is less because there is more of it and you increase the diameter, why doesn't that nullify the reduction due to the extra thickness?

shawnino

duplicate post somehow; mods please delete (or tell me how to do it and I'll clean up my own mess, thanks)

JJ_SO

The easiest way to find out is setting some primes and some zooms to the same aperture and shoot all of them wth the same shutter speed under the same light conditions

haroldp

@haroldp: the Sigma is not telecentric. Telecentric lenses are very specialized lenses for machine vision and measuring applications. The ones I worked with are object side telecentric. This means that the object does not change its size in the image within a certain range. These lenses usually have a fixed working distance and the diameter of the front element must be at least the size of the object. So I think one of us misunderstood something about either the lens or the meaning of tele centric.

A telecentric lens is one whose optical center, the point from which 'focal length' is measured, is actually behind the rear element.

Any full frame SLR lens with a focal length less than 58 mm is of necessity telecentric, otherwise it would not clear the swinging mirror. A 35mm lens for an FX camera must be telecentric if it uses physical material such as glass.

One reason non SLR cameras (such as Leica M) lenses had (and still have) an advantage at shorter focal lengths (< 58 mm ) is that they can use far simpler optical designs (such as double gauss) and not worry about clearing a mirror because there is no mirror. Every parameter a lens designer has to acommodate makes other corrections more difficult, or increases size / weight / complexity. A Leica 35mm Summilux easily outperforms any of these SLR lenses at F1.4 and is 1/3 the size and weight of the Nikon or Sigma equivalent, with fewer elements and glass air surfaces.

Another benefit of non-telecentric design is that the diaphragm can be at, or close to he optical center which usually make bokeh simpler and more pleasing.

It is no accident that Leica lenses at 58mm and less generally outperform SLR lenses, but at longer focal lengths this advantage no longer exists and Nikon / Canon tele primes are the equal of any available commercial lenses at their focal length / aperture. Military and specialized industrial optics are of course in a separate class.

I of course agree that an f-stop (eg: f2.8) is defined as the ratio between aperture and focal length and 'should' pass the same amount of light regardless of focal length except for what is absorbed / reflected by glass and coatings, which is where t-stop comes in.

Exposure should be about the same, I expect some loss in the more complex design, but a full stop as claimed is probably a test condition artifact.

DOF on the other hand can be affected by the location of the aperture, but again a full stop still seems high, but more plausible.

Regards ... Harold

Correlli

A telecentric lens is one whose optical center, the point from which 'focal length' is measured, is actually behind the rear element.

Strange, I know this type of lens construction as "retro focus"...

@Correlli the f/stop is not a dimension for light transmission. It's the relation of aperture to focal length and therefore the writing style f/2, i.e.

It doesn't say anything to the amount of light passing through the aperture. If you use a grey-filter, the aperture doesn't change, either. Lots of glass inside a lens is nothing else than a sort of grey filter.

That is what I have been trying to say. Depth of field should be about the same, but transmission of light (image brightness) could be different - although one stop seems to be a bit much for these lenses. The lenses I tested back then did show a difference of app. one f-stop in light transmission (so I would have to open the zoom one f-stop more to get the same exposure). But that was 20 years ago...

I learned that with "regular" glass each glass-air surface will cost you about 5% transmission. By using specialized glass this can be reduced. Especially surface coatings can reduce the loss significantly.

JJ_SO

That's correct, but for instance Nano-coating is usually only used on one or two elements - not the whole lens with a dozen elements.

haroldp

Strange, I know this type of lens construction as "retro focus"...

You are correct, telecentric maintains the same magnification over distance.

My content was correct even if using the wrong term.

regards .. H

Symphotic

Hello, Simba:

Since we are dealing with f/ stops there is an easy check for the proper function of you lens: visual inspection.

The transmission change in zoom over the 35 is not likely to cause a full one stop change in the shutter speed, but an incorrect f/stop setting would. I had a similar problem recently where the f/stop reading was off on a lens with a damaged weather seal and dirty pins.

The simple test for you is to set the zoom lens at f/2.8, look through the viewfinder and press the depth of field preview button. If your electronics are faulty, you will see the image get slightly darker and it is actually stopping down to f/4. It is possible the lens could be registering aperture incorrectly, which would give these results. (it could also be that your Sigma lens is NOT stopping down to f/2.8)

Try cleaning the contact pins in your camera and on your lens, and if that doesn't fix it, a quick run to Nikon service will clear it up. Otherwise, the problem needs additional study.

JJ_SO

Always interesting to test conclusions afterwards. I tried 6 or 7 lenses, all at f/5.6 and all at 1/20sec quickly one after another. Top row left: 14-24 at 24mm, 105 Micro.

Bottom row: 85 mm, 1/10 left f/5.6, right f/8 to get an idea how much 1 f/stop actually looks like-

What do you think? If it's not a full f/stop, it's maybe 1/2 or 2/3? So, 1 f/stop appears to be possible.

tc88

Postings starting from John all provided very helpful insights. Here are my 3 cents, building on top of what people already said.

Light loss due to extra glass. I remember reading somewhere light loss on optical glasses used for lens to be about 1.5% per element. Sigma 35 f/1.4 has 13 elements, and Nikon 24-70 has 15 elements. As several posters mentioned and I agree, I doubt that has much to do with it.

Vignetting behavior. As spraynpray mentioned, larger lens/glass diameters should be used with larger aperture. However, here are two things to be considered. (a) lens are made/sold with aperture specification only, so two lens can have same aperture but very different glass sizes. (b) bigger glasses are more difficult to manufacture and design since the additional light paths have to converge to the same point to maintain sharpness. Thus I can see manufacturers "cheat" in this regard, and why some lens (especially low end ones) have heavy vignetting wide open. Nikon 24-70 is more top end though. I just checked photozone and they reported loss of 1 stop vignetting at f/2.8 for 24-70. I don't know if you use the hood, that may affect something furtuer. Note, that value is only at the corner, though it could affect your metering somewhat.

DoF. I have a feeling that haroldp may be right in this regard. The distance from the aperture to the sensor may affect DoF. If indeed true, that's something interesting to consider between lens.

Simba, thanks for bringing an interesting topic. A lot of times things are not as simple as they apear. JJ_SO, thanks for the testing. Can't see the images though.