Why I dont like fixed aperture zoom lenses..

heartyfisher

Just continuing the thread from the old forum..
http://nikonrumors.com/forum/topic.php?id=27874

Heh, I pointed out that if you leave it on f/4 it is constant. Consider the 2.8 at the wide end a bonus. There was a whole thread on why constant aperture lenses couldn't be variable at the wide end, since it was "in there" anyway. The trade off is image quality, so keeping it stopped down to f/4 gives good quality, low weight, and a standard zoom range. That's the theory anyway since nobody has apparently seen the Sigma AF 17-70mm f/2.8-4 DC HSM OS macro in the wild yet. BTW, you can get the non-OS version and shave 50grams off the weight, gets you to like 475grams. Not too shabby.


Remember this discussion? http://nikonrumors.com/forum/topic.php?id=27874

This sigma is the proof that you can build an f/4 lens but allow it to be opened up at the wide end, with the resulting loss in quality. Its there if you want it, otherwise keep it on f/4.

By that logic the 18-55 is a constant aperture lens if you set it to f/5.6

Let's change that to another example.

The error in thinking that the 70-200 f/2.8 with it's (obviously) 71mm physical aperture can't instead be a 70-200 f/1->f/2.8 is in believing that there are large enough elements to do 70 f/1. The same is true of most all the other constant aperture lenses. They may have the physical aperture diameter for being a faster lens on the wide end, but they don't have the glass where it's needed. It's not just about the front element, it's about the entire path.

There are plenty of people who would give their left leg for a 70 f/1 - even if it came vignetted and aberated out the ying-yang. It isn't being withheld due to marketing. It's being withheld due to physics.

No one is saying that 70-200 f/2.8 will be 70 f/1 if aperture is fully opened. The final aperture returned to the camera is supposed to also be calibrated using the amount of the light that goes through. Otherwise, your 24-85 f/3.5-4.5 will become 24-85 f/1.5-4.5. However for example, I would guess that 70-200 f/2.8 may perform reasonably at 70 f/2.4.

It will be sharper at f/2.8 compared to f/2.4, but same can be said that it's sharper at f/4 compared to f/2.8.

There can be possible vignetting at 70 f/2.4, but many lens have similar problem.

It will take in more light, so it will be faster.

Another way to look at it, the opening at 70 is clearly smaller than the opening at 200. Since physically it can be opened bigger, and let's say it's opened slightly more to 70 f/2.7. Does anyone expect that the performance falls off the cliff that it's not usable? I would think not. So here you have it, all zoom lens are really variable aperture lens.

There can be many arguments to support a fixed aperture lens, mostly for user convenience and to prevent user screw up. Those are valid reasons, but not exactly related to the physical limitations.

heartyfisher

Keep in mind that Nikon (and most SLR lens manufactures) use F stop values in lenses, not the absolute value of the light transmitted (Tstops) through the glass. The chances of a F2.8 zoom lens being F2.8 throughout the entire range is impossible, due to the physical characteristics there of. There are simply too many variables to create a true constant aperture zoom. The aperture given is an average of what can be expected throughout the range (focused at infinity).

Thats really interesting .. didn't think of that .. so there is physically really no fixed aperture zoom..

PB_PM

To a point, the actual aperture value of say the 70-200mm F2.8, would likely be T2.7-T2.9 or something like that. Although the lens isn't really F2.8 or consistent, it is close enough.

Msmoto

The actual effective aperture of most lenses is far less than the f/stop seen on the lens. If one checks the DxOMark tests, one sees for example, an f/1.4 lens may actually have the transmission of f/1.7, An f/2.8 lens may be f/3.1 or 3.2. The reason is, as I understand it, the loss through the multiple elements, internal reflections, coatings absorbing some light, and other factors.

Emcee

I think this discuss is moving off topic and comparing apples and oranges. T stop (transmission) and f stops are completely different.

Two lenses can have the same f numbers but transmit totally differing amounts of light.

PB_PM

That is the point, and why we were talking about T stops in the first place.

Ironheart

So TTL metering will sense the actual amount of light that is coming through the lens (hence the name). Based on that it will pick an f/stop, a shutter speed, and potentially an ISO (in all but manual mode), and close the aperture to the "correct" size. Is this why some lenses tend to under or over expose? Because f/4 ain't really f/4?

PB_PM

Ironheart, that is exactly what is happening.

Emcee

So TTL metering will sense the actual amount of light that is coming through the lens (hence the name). Based on that it will pick an f/stop, a shutter speed, and potentially an ISO (in all but manual mode), and close the aperture to the "correct" size. Is this why some lenses tend to under or over expose? Because f/4 ain't really f/4?

But to this point, wouldn't the camera meter according to the light it receives? So effectively if less light is coming in it would compensate by choosing a higher exposure value? This isn't a factor in manual of course because the shooter is then controlling everything but in any other mode the camera is trying to achieve middle grey regardless of the lens. No?

PB_PM

As far as I know, digital cameras meter based on the F-Stop value, at either 1/2 or 1/3 of a stop, depending on settings. If the value is smaller than that, slight over or underexposure could occur, with some metering modes (Matrix, I'm pointing at you!).

heartyfisher

So TTL metering will sense the actual amount of light that is coming through the lens (hence the name). Based on that it will pick an f/stop, a shutter speed, and potentially an ISO (in all but manual mode), and close the aperture to the "correct" size. Is this why some lenses tend to under or over expose? Because f/4 ain't really f/4?

Yes and no.. you are basically correct .. but its no excuse for a lens to underexpose or over expose. as the T-Stop should be consistent whatever the f-Stop is set to be. The metering is done at the wide open setting which is say F2 and T2.2 so if the metering says it should now set the exposure to F4 to take the photo then the T-Stop will then be T4.4 so the percentage difference should always be the same, if the metering is accurate. So the issues with the lens underexposing or overexposing is a result of the aperture blade mechanism not being calibrated correctly and not to do with what we are discussing now which is the fact that Zooming changes the T-STOP as it zooms.

For example if you set the aperture of a lens at the wide end of the zoom to F2.8 the T-stop may be T3.0. and at the long end it may be T3.3 so although its a fixed aperture zoom the exposure time will need to be changed between the wide end and the long end.

Which in our thread of discussions mean that the argument for the advantage of having a fixed aperture zoom of being constant exposure from wide end to long end is really a false assertion and we should in fact not need fix aperture zooms as it really does not provide that advantage.

heartyfisher

To a point, the actual aperture value of say the 70-200mm F2.8, would likely be T2.7-T2.9 or something like that. Although the lens isn't really F2.8 or consistent, it is close enough.

My understanding of the T-Stops is that they are always higher than the F-stop. The Fstop is what the aperture diameter should be for the required Focal length. The T-stop is the measured the amount of light that actually gets through.

T stops in SLR/DSLR with TTL metering is really a non factor since the light is measured Through The Lense. Tstops were used in cinematography. They really needed very accurate metering because they filmed the same scene from various angles with different lenses. The results are then spliced together and changes in exposure would be a big distraction to the full scene. Note that they had very little flexibility in shutter speeds and ISO sensitivity.


In DSLR TTL metering, the actual T-Stop is really irrelevant to the in camera metering accuracy. Setting the lense to F2.8 the actual TSTOP may be 3.2. In macro lenses setting the Fstop to F2.8 at macro distances with 1:1 magnification could have TSTOPS of up to T5.6 or even T8. The metering will still be accurate.

Golf007sd

Ok guys some of all this T-Stop disccusion has gone over my head and maybe other as well. So let me first share with everyone what I have found out so far:

T-Stop: "In practice, even the best lenses exhibit light absorbance, effectively “stealing” some of the light going through them. This means that if you calculate the exposure based on the f-stop of the lens, you will end up underexposing the image, because less light is reaching the film plane than is expected in theory. T-stops are the f-stop of the lens corrected for its absorbance and reflectance. The T-stop is the true speed of the lens, calculated by compensating for its light absorbance and reflectance, and will result in accurate exposure."

With respect to lenses, I found this post.

Even with all this info, I still think this is more for the techies and "pixel peppers." I much rather see the final result by the photographer and forget all the tech talk. Show me a beautiful image and if I like it I will give you big high-five and have you tell me about what you had to do to get that shot.

heartyfisher

Ok guys some of all this T-Stop disccusion has gone over my head and maybe other as well. So let me first share with everyone what I have found out so far:

T-Stop: "In practice, even the best lenses exhibit light absorbance, effectively “stealing” some of the light going through them. This means that if you calculate the exposure based on the f-stop of the lens, you will end up underexposing the image, because less light is reaching the film plane than is expected in theory. T-stops are the f-stop of the lens corrected for its absorbance and reflectance. The T-stop is the true speed of the lens, calculated by compensating for its light absorbance and reflectance, and will result in accurate exposure."

With respect to lenses, I found this post.

Even with all this info, I still think this is more for the techies and "pixel peppers." I much rather see the final result by the photographer and forget all the tech talk. Show me a beautiful image and if I like it I will give you big high-five and have you tell me about what you had to do to get that shot.

LOL! Go back to the PAD Golf (:| Leave us Mad people ) to our madness ! 8-} <:-P

[Deleted User]

This is all fascinating...but once again I can't help feeling some of us are missing the point.

Let me put this another way, does anyone here actually take photographs any more? I'm obviously asking this 'tongue in cheek' but I do get worried when I see people spending more time inspecting the theory than actually benefitting from the practical...

spraynpray

)

You read my mind! We all need to get out more - in both senses of the expression.

Paperman

For those who are still not lost/bored ...

The subject to go to is " hidden ISO Boost" by camera manufacturers, discussed a year or two ago in this forum.

Anything from a quarter to half stop is "discreetly" added to our exposures ( with fast lenses @ wide open apertures ) by ISO boost which is NOT shown in the EXIF . The DSLR knows which lens/what setting it is ( thanks to contacts ) and its Tstop loss and then then boosts ISO to match correct exposure. ( You think you are shooting ISO 100 but it may be set to ISO 140 " internally ".

Same story with Sony Translucent mirror SLTs ; 1/2 to 2/3 a stop is compensated by ISO boost , as Sony has disclosed.

You wouldn't see any of this ISO boost in the EXIF.

As I said, it is accepted to be true for fast lenses at wide open apertures but I'm not sure if the ISO boost / compensation is done for every lens at every aperture setting. I doubt it.

TTL metering is not of any relevance/ does not solve the problem as not only the exposure but the data we see in EXIF has to match what the camera is set at IN EVERY MODE . ( You can't have different metering values for M and Auto modes or different readings for different lenses )

TaoTeJared

So TTL metering will sense the actual amount of light that is coming through the lens (hence the name). Based on that it will pick an f/stop, a shutter speed, and potentially an ISO (in all but manual mode), and close the aperture to the "correct" size. Is this why some lenses tend to under or over expose? Because f/4 ain't really f/4?

Yes and no.. you are basically correct .. but its no excuse for a lens to underexpose or over expose. as the T-Stop should be consistent whatever the f-Stop is set to be. The metering is done at the wide open setting

"Why some lenses tend to under or over expose" - I have to go with No. To notice a difference the image would have to be at least 1/3 off, which is much more than what apertures are allowed to be off by. These are all tested in Nikon's factory, if it doesn't make the cut, it gets kicked out - at least for the better lenses.

Almost there on the flash- maybe it has changed some from the old days. First, there is a difference between TTL and iTTL which we all use with Nikon Flashes. The whole "i" thing adds much more to TTL but I'll be damned if I don't have to read the manual each time to really understand what it added.

The main thing I do know is with iTTL, from what I have read, the amount of flash and other settings are achieved when the lens is stopped down to take the photo. Yes it happens in less than 1/10,000 of a second.

All of the t-stop transmission, yes interesting but has zero to do with anything for exposure or a variable you can do anything about it or should try to.

Msmoto

@ TaoTeJared

And, I was thinking I was the only person who had to read her manual to find out what is going on

I agree, TTJ, a lot of folks discuss all this stuff, and the primary topic of the thread is somewhat esoteric IMO. If a piece of equipment works, use it, get the photos to the final and post. Some days I am still trying to figure out what that button in the middle of the on/off switch is..... :-))

soap

But to this point, wouldn't the camera meter according to the light it receives? So effectively if less light is coming in it would compensate by choosing a higher exposure value? This isn't a factor in manual of course because the shooter is then controlling everything but in any other mode the camera is trying to achieve middle grey regardless of the lens. No?

No. They meter with the lens wide-open. If the T to F-stop offset of the lens is non-linear they will miss and overexpose on lenses which have significant vignetting only when wide open.

As to the "behind the scenes silent ISO boost" situation. The only place I've seen that documented is in compensation for the poor oblique-angle photon reception of photosites - ie boosting ISO at f/1.4 vs f/2.0 to compensate for the fact that many the additional photons are uncollectable by a 3D photosite. While it is completely imaginable that Nikon et al use this technique also for the correction of metering through a highly vignetting lens I haven't seen where it's been shown they do - but would love a link.

EDIT 1: Testing such a thing should be easy. Since vignetting has near no effect on the center of the image one could spot meter a perfectly neutral field (thus avoiding the lens "flaw") and compare the results to Matrix metering the same field (since Matrix would weigh the darkened edges).

EDIT 2: If you have any doubt that spot and matrix would produce different results on a neutral field you could test also with a lens known not to vignette much as a control.

Paperman

I don't wan't to ad the link here as sometimes they are removed but you can search it as " Mark Dubovoy Luminous Landscapes open letter " . The article is based on Dxo Lab Tstop test results for some fast lenses.

soap

Right, and that is the one which talks about compensation for oblique photons, not t-stop non-linearity. The chart at the end shows the gain decreasing to zero by f/2.8.

The issue I was talking about would show up as overexposure at small apertures due to the error in metering through a vignetted large aperture.

Fraucha

@darkside, Yes people still take photos. They test their theories out by taking high res photos of their cats.

PB_PM

Haha, lucky for me I don't have a cat (or any pet for that matter).

Golf007sd

@Fraucha Wait...wait...wait a minute here :-t

Dog & Cat rule! Thank you very much...now back to our regular programing.

Play ball...

[Deleted User]

@darkside, Yes people still take photos. They test their theories out by taking high res photos of their cats.

If you're talking about a photo published on my personal blog to illustrate lens quality, then yes, I once took a photo of my cat... but you didn't see that here, so why mention it?