DX Lenses should be brighter/faster than FX lenses

WestEndFoto

Before we drink too much of the DX koolaid, remember that 1.8 on DX is about the same as 4.0 on FX.

spraynpray

@Snakebunk: Pistnbroke is talking about the Samyang/Rokinon/Bower/etc 14mm f2.8 manual focus lens.

Before you get too carried away with the Sigma lenses I would make sure you are happy with the low light focussing accuracy. I wasn't.

snakebunk

@spraynpray: I have not had any problems. Have used the Sigma 50/1.4 A indoors wide open in low light, on my D800. We have discussed this earlier but isn't low light focusing dependent on the camera rather than the lens? I mean the lens is only to focus as the camera tells it to, right?

haroldp

Current lenses have focusing motors in the lens.

spraynpray

@snakebunk: I was using it on my D750 which isn't too shabby in low light. I was shooting a big party and was having to shoot at f1.4-f2 up around ISO12800 and it gave me too many misses.

snakebunk

Ok, if it wasn't good enough I can't argue. I still don't understand though how a lens can be better or worse in low light. It should be the same regardless of light and only be better or worse at performing the focus that the camera tells it to.

spraynpray

Exactly. The camera says focus there but the lens doesn't make it.

snakebunk

Exactly. The camera says focus there but the lens doesn't make it.

But then it should be equally bad regardless of light. Don't you agree?

Ironheart

Before we drink too much of the DX koolaid, remember that 1.8 on DX is about the same as 4.0 on FX.

Before we jump to conclusions, can you clarify your statement? In what way? The way I figure, DX gives a one stop DoF advantage meaning of course deeper.

Ironheart

Exactly. The camera says focus there but the lens doesn't make it.

But then it should be equally bad regardless of light. Don't you agree?

For Nikkor lenses, yes. For third-party all bets are off. SLR AF systems are finicky, heuristic, and finely tuned complicated physical devices. Each Nikkor lens has a unique map of how it responds and the protocol has to be reverse engineered for everyone else. At the margins, I could easily see the "hit rate" start to drop off.

Rodger at lesrentals has some excellent articles with the gory details. In a nutshell SLR AF systems aren't as accurate as you think they are. They operate within a range of acceptability, and shot to shot accuracy is like a good marksman, a tight grouping, but never really hitting the same spot twice.

WestEndFoto

Before we drink too much of the DX koolaid, remember that 1.8 on DX is about the same as 4.0 on FX.

Before we jump to conclusions, can you clarify your statement? In what way? The way I figure, DX gives a one stop DoF advantage meaning of course deeper.

I was thinking of the title of the thread. DOF, ISO.

Paperman

Rodger at lesrentals has some excellent articles with the gory details. In a nutshell SLR AF systems aren't as accurate as you think they are. They operate within a range of acceptability, and shot to shot accuracy is like a good marksman, a tight grouping, but never really hitting the same spot twice

Note Admin - Italic button not working. Above paragraph in Italics actually

I always say it ... Everyone is expecting too much of AF systems. Just think of how small an area on the sensor the AF system is trying to read info from ( how large is that bird's eye / coverage of a single AF point on the actual sensor ) and then re-evaluate what performance can logically be expected from auto focusing. There is probably always an error which we do not notice because it is covered by DOF or so. I believe any AF system is doing hell of a job - it is actually like magic - when compared it to what we can do manually.

starralazn

@DaveyJ, i think i go to different kinds of events, i see plenty of primes at anime conventions. although i suppose theres a different focus to capture different moments with differing types of subjects if you're explicitly an event photographer(one who tries to capture the spirit of the moment).
i have been in the situation where i was asked to capture some of the general atmosphere of the event, and brought a 24-70 2.8 for safety, however, i found myself wanting to use a 50mm for >75% of the shots... just my limited perspective.

snakebunk

Exactly. The camera says focus there but the lens doesn't make it.

But then it should be equally bad regardless of light. Don't you agree?

For Nikkor lenses, yes. For third-party all bets are off.

Are you saying that a Nikkor lense can move the glass regardless of light but Sigma lenses may need light to move the glass? This is very strange to me (it is like the Sigma lenses use solar power to move the glass).

My logic and experience tells me that low light focusing capability is only dependent on the camera. If the lens has a focusing problem it remains the same regardless of the light.

Thanks for the article, I promise to read.

Ironheart

Exactly. The camera says focus there but the lens doesn't make it.

But then it should be equally bad regardless of light. Don't you agree?

For Nikkor lenses, yes. For third-party all bets are off.

Are you saying that a Nikkor lense can move the glass regardless of light but Sigma lenses may need light to move the glass? This is very strange to me (it is like the Sigma lenses use solar power to move the glass).

My logic and experience tells me that low light focusing capability is only dependent on the camera. If the lens has a focusing problem it remains the same regardless of the light.

I'll dig up the link for Rodger's articles, rather than trying to describe here how they work.
I am not saying anything like that. What I'm saying is that Nikkors will perform better across the entire range of light. At the margin, I have also observed, as have others, that third party lenses (and for that matter a Nikkor that needs service) will get inconsistent, sooner that it should. AF isn't binary, either working, or not working. It tends to degrade as the light falls.

http://www.lensrentals.com/blog/2010/07/how-autofocus-often-works
http://www.lensrentals.com/blog/2014/02/why-you-cant-optically-test-your-lens-with-autofocus
http://www.lensrentals.com/blog/2012/07/autofocus-reality-part-1-center-point-single-shot-accuracy

I really recommend all 4 parts of the last article, but these should be enough to burst anyone's autofocus bubble )
I agree with @Paperman, it's a wonder it works at all....

WestEndFoto

Rodger at lesrentals has some excellent articles with the gory details. In a nutshell SLR AF systems aren't as accurate as you think they are. They operate within a range of acceptability, and shot to shot accuracy is like a good marksman, a tight grouping, but never really hitting the same spot twice

Note Admin - Italic button not working. Above paragraph in Italics actually

I always say it ... Everyone is expecting too much of AF systems. Just think of how small an area on the sensor the AF system is trying to read info from ( how large is that bird's eye / coverage of a single AF point on the actual sensor ) and then re-evaluate what performance can logically be expected from auto focusing. There is probably always an error which we do not notice because it is covered by DOF or so. I believe any AF system is doing hell of a job - it is actually like magic - when compared it to what we can do manually.

This reminds me of when I bought my first Nikon. It was a scope for my 7mm Remington Magnum. I was 9 years old and it bothered me that I could not get single hole groups at 100 yards with my custom built $2,500 rifle when my dad could with his 7mm Shooting Time Western.

So I upgraded my Bushnell to a Nikon and noticed right away how much clearer the target was. I even started hitting the six inch gong at 1,000 yards consistently and by the time I was 16 I was one of the top marksman in the province.

Needless to say, I have been loyal to Nikon ever since. Even my binoculars and the lenses in my glasses are Nikon.

heartyfisher

Before we drink too much of the DX koolaid, remember that 1.8 on DX is about the same as 4.0 on FX.

Where did you get that from? my understanding is equivalent DOF = Aperture X DxMultiplier ie 1.8 x 1.5 = 2.7

That is just the DOF, you still get all the advantage of the bright 1.8 aperture for
1) Bright Viewfinder viewing.
2) AF sensitivity and efficiency
3) Lower ISO settings ...
4) Faster shutter speeds.
5) Heavier lense -- some like the heft and momentum stability
6) Larger front element ... compatibility with other lenses ( I never understood this logic but ... )

PS: Actually having a fast portrait lense with a deeper DOF may be a good thing .. no?

WestEndFoto

Before we drink too much of the DX koolaid, remember that 1.8 on DX is about the same as 4.0 on FX.

Where did you get that from? my understanding is equivalent DOF = Aperture X DxMultiplier ie 1.8 x 1.5 = 2.7

That is just the DOF, you still get all the advantage of the bright 1.8 aperture for
1) Bright Viewfinder viewing.
2) AF sensitivity and efficiency
3) Lower ISO settings ...
4) Faster shutter speeds.
5) Heavier lense -- some like the heft and momentum stability
6) Larger front element ... compatibility with other lenses ( I never understood this logic but ... )

PS: Actually having a fast portrait lense with a deeper DOF may be a good thing .. no?

Sorry, that is what I get when I try to do math and walk (down the street) at the same time - on a Blackberry to boot. Your math is correct. 1.8 is about the same as 2.7.

Regarding the ISO, the one stop improvement in light gathering ability, when it comes to noise, is negated by the general one stop disadvantage of a DX sensor.

heartyfisher

So the trade of is ... lets do a comparison
D5 + 70-200 VR F2.8 vs D500 + 50-100 F1.8

1) Noise Sensor IQ the same - bec the D500 can run at 1+ stop ISO lower due to the faster aperture.
2) AF - Advantage to DX bec of Brighter aperture.
3) VR - advantage to FX the FX lens has VR
4) Subject Movement - Advantage to DX bec of faster shutter speed available.
5) DOF - advantage to DX bec of deeper DOF esp at low apertures ... ;-) ( actually no difference ;-) )

So Overall DX comes out ahead ... ;-)
( not mentioning the Price difference ;-) , Weight. etc...)

Ironheart

Before we drinking too much of the DX koolaid, remember that 1.8 on DX is about the same as 4.0 on FX.

Before we jump to conclusions, can you clarify your statement? In what way? The way I figure, DX gives a one stop DoF advantage meaning of course deeper.

Yeah, that's what I was getting at here too.
However, you are both wrong. f/1.8 DX gives the same DoF as a f/2.45 FX. The only way to get the right answer is to use a DoF calculator. BTW, it turns out it is one stop, like I said.

heartyfisher

Before we drinking too much of the DX koolaid, remember that 1.8 on DX is about the same as 4.0 on FX.

Before we jump to conclusions, can you clarify your statement? In what way? The way I figure, DX gives a one stop DoF advantage meaning of course deeper.

Yeah, that's what I was getting at here too.
However, you are both wrong. f/1.8 DX gives the same DoF as a f/2.45 FX. The only way to get the right answer is to use a DoF calculator. BTW, it turns out it is one stop, like I said.

Ok so a 1.8 DX lense (say 100mm) has shallower DOF than a 2.8 FX lens at equivalent Focal length (150mm)?

Ironheart

Before we drinking too much of the DX koolaid, remember that 1.8 on DX is about the same as 4.0 on FX.

Before we jump to conclusions, can you clarify your statement? In what way? The way I figure, DX gives a one stop DoF advantage meaning of course deeper.

Yeah, that's what I was getting at here too.
However, you are both wrong. f/1.8 DX gives the same DoF as a f/2.45 FX. The only way to get the right answer is to use a DoF calculator. BTW, it turns out it is one stop, like I said.

Ok so a 1.8 DX lense (say 100mm) has shallower DOF than a 2.8 FX lens at equivalent Focal length (150mm)?

Using your example:

100mm f/1.8 DX subject 4m away DoF=10.3mm
150mm f/2.8 FX subject 4m away DoF=11.5mm
150mm f/2.5 FX subject 4m away DoF=10.3mm

I got the f/2.45 by using 35mm and 50mm which are not quite equivalent, so f/2.5 is more accurate, exactly one stop on the 1/3 stop scale.

9viii

So the trade of is ... lets do a comparison

4) Subject Movement - Advantage to DX bec of faster shutter speed available.

Motion blur should be no different as long as your field of view is the same.

I will agree that you potentially get an AF advantage though, running a cropped 300f2.8 instead of 400f4 should get you faster and more precise AF. Those 300f2.8 lenses tend to be some of the best performing in any given category.

heartyfisher

"Motion blur should be no different as long as your field of view is the same." ??

You may be assuming the shutter speed is the same .. the argument is because the aperture is wider we can get a faster shutter thus reduce motion blur.

Msmoto

I am always fascinated by DOF discussions, as in my experience, the only significant differences occur when looking at f/1.8 vs. f/5.6, or at least several f stops difference. My perfectionistic issues, not always good, lead me into the thinking that DOF is in almost all cases, much less than any calculations, and I attempt to focus on what I think is the key to producing a good image.

In terms of differences in DX and FX lenses, I have been using all FX lenses for sometime now, with one exception, my modified 10.5mm. In the days of shooting 4" x 5" and larger sheet film, we used many of the same lenses for several formats.

About the only difference in lenses is the maximum image circle. If this is smaller, most likely the lens will be less expensive and that is about the only conclusion I can make. All other characteristics are of little consequence....

It is accepted, I think, that most professionals are shooting full frame, and this influences the market for more expensive lenses. The crop sensor market is generally determined by those whose budget restraints require less costly lenses. It makes a lot of sense marketing wise to produce some lenses usable in both markets, thus, again IMO only, the much greater number of full frame vs. crop sensor lenses. The slower lenses, less costly, thus often the "kit" lens.

oh well...