[Aristophanes]

A 17-50 f2.8 lens will give you the same exposure on APS-C as a 28-75 f2.8 would give you on full frame, but with a stop more depth of field.

To be precise, a larger default circle of confusion.

[cali92rs]

they might be cheaper and lighter than if they were made to cover a larger format

They might be...haven't been the case with any camera manufacturer I am away of.
Lighter, maybe....cheaper, not so much.

[Kunzite]

That's why I said "they might be", and not "they are"; those are possible reasons to prefer an APS-C lens over a FF one (assuming the same mount). I would expect such differences might appear e.g. for wide zooms; for example the Canon EF 16-35 is significantly more expensive than the EF-S 17-55 (both f/2.8) - and they are about the same size despite being a 2.2x vs. 3.2x zoom.
In practice you won't find often two lenses with the same focal range and max aperture - for APS-C and FF - to compare.

[ElJamoquio]

That's why I said "they might be", and not "they are"; those are possible reasons to prefer an APS-C lens over a FF one (assuming the same mount). I would expect such differences might appear e.g. for wide zooms; for example the Canon EF 16-35 is significantly more expensive than the EF-S 17-55 (both f/2.8) - and they are about the same size despite being a 2.2x vs. 3.2x zoom.
In practice you won't find often two lenses with the same focal range and max aperture - for APS-C and FF - to compare.

Dr. Camera: APS-C lenses aren't smaller, aren't lighter, and aren't cheaper.

[Kunzite]

There are many things wrong with that article, but I'll say only this: I am talking about using the lenses on the same (APS-C) camera (answering TzalamChadash's questions).

[ElJamoquio]

An aps-c lens on an aps-c sensor is almost always cheaper than a ff lens on a aps-c sensor, yes.

[jsherman999]

Well, this is interesting (from dpreview's RX10 review)



(I saw where jogiba linked the above in this thread, but I cant seem to find the section of the Rx10 review where that image appears...)

Anyway the info above is exactly what places like dpreview should include.

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[Pål Jensen]

Anyway the info above is exactly what places like dpreview should include.

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Isn't the figure wrong? It not equivalent aperture but equivalent DOF wide open. These are two different concepts. If they don't separate the two, it proves they haven't a clue.
However, thiness of DOF wide open has no relevance for people who are not concerned about DOF wide open; it is a purely subjective issue what DOF wide open you are happy with. People may want fast lenses to shoot in low light. There's no correlation between wanting to shoot in low light and wanting thin DOF (arguably the opposite). Why should DOF wide open be a benchmark?

Is the DOF scale of a FF lens with certain angle of view more relevant than the DOF scale for an APS lens with the same angle of view (on an APS camera) and speed? Why?
And why is it it is only DOF wide open at infinity thats important and not DOF wide open at the close focusing distance for "equivalent" lenses?
And why isn't DOF range: ie DOF possibilities important? Do everyone shoot exclusively wide open all the time?

In addition, lenses claimed to be equivalent after this flawed "law" are patently not. I can shoot images with an "equivalent" APS lens you cannot with a "equivalent" FF lens wide open (not to mention stopped down) both in DOF, magnification and perspective due to the laws of optics (which are relevant - nedless to say); what kind of equivalency is this and for whom?

Exposure is relevant for all photographers. It is objective, Not subjective as DOF wide open equivalency at restriced focusing distances.
The true equivalence is therefore numbers printed on the lenses (that's why they print it there - believe it or not!). Thats why I can mount a Pentax645 FA 45/2.8 on any camera it will fit and know exactly what it does; angle of view- and DOF-wise. Thats equivalency.
Theres no 200 year old conspiracy from the manufacturers.
Different formats are not DOF equivalent. Better get used to it.

[eyeswideshut]

Isn't the figure wrong? It not equivalent aperture but equivalent DOF wide open. These are two different concepts. If they don't separate the two, it proves they haven't a clue.

Hush, it was quiet on the equivalency front for a few days....
P.S. Parallax is watching you!

[Andi Lo]

The easiest way to see how it works is to compare it in real world using a lens / camera combination that you use often. Borrow an "equivalent" bigger sensor format. You'll see the difference rightaway if you're used to how that focal length / aperture combination behaves

I know that practically 75mm/2.8 on FF looks exactly the same as 50mm/2 on APSC. I really couldn't care less what the equivalence math is, because I know that my pictures look the same.

This means with FF I can use a zoom (28-75/2.8 and 70-200/2.8), to shoot as if I was using a 50mm lens on APSC at f/2. It also means that I can pay $400 for a 85/1.8 to shoot like a 55mm lens on APSC at f/1.2 (and with AF!).

I know not everyone cares about shooting at f/2 or f/1.2, and that's totally fine. Since I shoot 50mm f/2 all the time on APSC, this affects me a lot.

Sorry if I'm just beating up a dead horse, I thought maybe practical findings helps :x

[Pål Jensen]

I know that practically 75mm/2.8 on FF looks exactly the same as 50mm/2 on APSC. I really couldn't care less what the equivalence math is, because I know that my pictures look the same.

This means with FF I can use a zoom (28-75/2.8 and 70-200/2.8), to shoot as if I was using a 50mm lens on APSC at f/2. It also means that I can pay $400 for a 85/1.8 to shoot like a 55mm lens on APSC at f/1.2 (and with AF!).

Shooting at 1.8 is not equivalent to shooting at 1.2; neither within formats or between them. In fact. the theory of exposure has nothing to do with formats; It is independent on formats. DOF is not involved either.

The easy way illustrate the principle of exposure is to compare it to a bucket that need to be filled with water through a hose. The size of the bucket represent the sensitivity; small bucket = high ISO - faster to fill. Aperture is the diametre of the hose or how much you open the tap; The time it takes to fill the bucket is shutterspeed. Wide open tap (large aperture) shorter shuttersped. Small buckets are not equivalent to large ones either.

DOF is not involved; it is secondary issue like motion blur or freeze due to shutterspeed.

One need to separate between DOF and aperture. The former is self explanatory (Depth of field) the latter is physical number derivated from a formula based on measurments. There is no room for discussion on this issue unless you use your own private definition of aperture. Hence, there are no equivalent apertures apart from the numbers witten on the lens - ie a 2.8 lens is a 2.8 lens regardless of format. Shooting at different aperture is not equivalent; see theory of exposure. Equivalent DOF is a separate issue*.

Likewise one need to separate between Focal lenght and angle of view. The latter is self explanatory; the former is a physical measurement. Just like with aperture there is no eqiuvalent focal lenghts except for similar focal lenght. Ie the Pentax 645 75mm lens is roughfly equivalent in focal lenght to the 77 Limited. Format is irrelevant as it is not part of the equation just like for aperture.
There are equivalent angle of view but it is not the same as eqivalent focal lenghts.

*The lenses you mention are not even DOF equivalent. They don't have the same DOF range. In addtion the 50/2 lens has one more stop to choose from. They also only DOF equivalent wide open under certain circumstances as the wider lens used on APS will have greater maximum magnification (remember the crop factor). Under the theory of equivalence the APS lens can give images impossible on the FF body in both magnification, perspective and DOF with all apertures. Hence, they are anything but equivalent.

This is easiest to visualize with macro lenses. Most Pentax macro lenses goes to 1:1. Lifesize (1:1) is a constant indipendent of format (just like aperture and focal lenght). However, formats are not constant (obviously). This means that while the subject is constant the format increase. Hence a 1:1 macro lens for the Q is a totally different beast than a 1:1 macro lens for the Pentax 67.

[ElJamoquio]

Equivalence is awesome. I'm glad it's so easy to figure out what the differences between formats are.

[jsherman999]

Shooting at 1.8 is not equivalent to shooting at 1.2; neither within formats or between them. In fact. the theory of exposure has nothing to do with formats; It is independent on formats. blah blah blah.

Pal, as usual you're conflating exposure with total light, or pretending that exposure is all that could ever matter to a photographer.

Answer this very simple query:

If a 1''-sensor bridge camera with a 24mm (equivalent) lens and an f/1.8 maximum aperture were to shoot the same scene as a FF camera with a 24mm lens and max aperture f/1.8, both shot wide-open, same distance to subject, same shutter speed, same exposure - and say the sensors were equally efficient, perhaps cut from the same wafer - which image would have more noise? (I think you've demonstrated you know which image would have more DOF, but how about noise?)

And why would they even have different amount of noise, if exposure (f-stop, shutter speed) was the same and the sensors were equally efficient?





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[Andi Lo]

Shooting at 1.8 is not equivalent to shooting at 1.2; neither within formats or between them. In fact. the theory of exposure has nothing to do with formats; It is independent on formats. DOF is not involved either.

Thanks for the writeup Pal, however from my experience, and for my purposes, they're basically the same. I've actually tried it side by side numerous times. I shoot portraits with 50mm at f/2 APSC, and 75mm/2.8 on FF, seconds apart from each other; aside from the obvious difference in rendering between lenses, the images look indistinguishable to me. You mentioned the 50mm on APSC being able to open up further, but the 75mm/2.8 on FF can zoom to 28mm... or to 200mm depending on which zoom you choose!

Any other reason why they might not be "fully equivalent" isnt really relevant to me. I noticed in your write up you mentioned that DOF is secondary, to me DOF and BG blurriness is primary as it really makes or breaks the image. ISO and shutter speed is what secondary to me, as long as shutter is fast enough and the image is noise-free enough, it's good enough.

I understand what you're writing on how technically these two formats cannot be equal the way I think they are, but for my purposes my formula works. We just use our cameras very differently I think.

[jsherman999]

Thanks for the writeup Pal, however from my experience, and for my purposes, they're basically the same. I've actually tried it side by side numerous times. I shoot 50mm at f/2 APSC, and 75mm/2.8 on FF, seconds apart from each other; aside from the obvious difference in rendering between lenses, the images look indistinguishable to me.

And that's because they would be almost indistinguishable, although 50mm at f/1.8 or f/1.9 on aps-c would be a closer match - but you're seeing correctly, for all intents and purposes 50mm f/2 aps-c =~ 75mm f/2.8 on FF in terms of FOV, DOF.

(And with similar sensor generations, resulting image noise would be about the same too, assuming same shutter speed and an ISO bump with the FF combo to maintain brightness.)

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