sensor dictates 'actual' f-stop?

i dont remember where i read it (someplace here) where a member was commenting on the 17-55 f2.8 IS USM and stated that the equivalent FF lens was the 24-105 f4L

the comment went on to say that the numerical equivalent of the 17-55 focal length was 27-88. okay. not exactly but pretty close. close enough that i wont argue it...

the poster THEN stated the 17-55 f2.8 was equivalent to the 24-105's f4....

here is where im having some confusion. IF in fact the f2.8 on a crop sensor is the equivalent of an f4 on FF, does that mean if i put my 70-200 f2.8L on a FF body that i really only have an f4 lens? if i owned the f4 version would i then in reality have a lens with only an f6 maximum aperture?

i would that mean if i bought the 50 f1.4 (i did) and used it on my 7d, that i would GAIN in light gathering, say that of the older 50mm f0.95?

something doesnt add up for me...

Ltdave wrote in post #14982712
i dont remember where i read it (someplace here) where a member was commenting on the 17-55 f2.8 IS USM and stated that the equivalent FF lens was the 24-105 f4L

the comment went on to say that the numerical equivalent of the 17-55 focal length was 27-88. okay. not exactly but pretty close. close enough that i wont argue it...

the poster THEN stated the 17-55 f2.8 was equivalent to the 24-105's f4....

here is where im having some confusion. IF in fact the f2.8 on a crop sensor is the equivalent of an f4 on FF, does that mean if i put my 70-200 f2.8L on a FF body that i really only have an f4 lens? if i owned the f4 version would i then in reality have a lens with only an f6 maximum aperture?

i would that mean if i bought the 50 f1.4 (i did) and used it on my 7d, that i would GAIN in light gathering, say that of the older 50mm f0.95?

something doesnt add up for me...

Somebody's been feeding you with a lot of misinformation, to say the very least. There are folks who give wrong advice here because somebody taught them something that wasn't quite true or they misunderstood the truth.

What's your background with photography? Do you have enough experience with a 35mm film camera to have a feeling about the field of view provided with various focal length lenses at various distances? We'll go forward once you can answer this.

I think you are referring to f/2.8 has the same DOF on a crop as f/4 does on a full frame. Light gathering is not the same f/2.8 will let more light in than f/4 on any camera. F stop is a ratio of the mechanics of a lens. The DOF, FOV changes from crop to full frame (assuming identical framing)

^ Yeah, I think this was the actual intended reference. It sounds like maybe the person who told you this just misunderstood the reference.

I don't understand why so many people try to draw up aperture equivalents between sensors. An aperture of f/2.8 is f/2.8 no matter what camera body it is on. The same amount of light enters no matter which way you try to cut it. The lens doesn't suddenly gain an improvement in light gathering if you take it off a 450D and put it on a 5D3.

How the light is processed after passing through the lens may be different, but that has absolutely NOTHING to do with the lens, and everything to do with the technology of the sensor. Therefore, by claiming that a lens has better "light gathering" on a FF is just flat out wrong. Take that lens and put it on a 10yr old FF technology and compare the same lens on the 7D crop sensor...suddenly people would be claiming f/2.8 gathers more light on a crop body.

started shooting film in 1976 with a Canon EXauto...

moved to a Canon FTB ql in 1981...

moved to a Canon AE-1 in 1982...

added a Pentax K1000 in 1984 just because...

photo editor of college newspaper from 1984-1985 (3 semesters)...

staff photographer Gannett News Service newspapers 1985 (6? 7? months)...

worked in a custom photo lab 1983-1985...

still photographic specialist/graphic arts specialist USAF 1985-1993...

no one TOLD ME this information, i BELIEVE it was a comment in one of the threads regarding the 17-55mm f2.8 IS USM lens...

i fully understand the relation ship of DOF and the mathematical processes in determining the f-stop number of a lens. i understand that f-numbers are the powers of the square of 2...

i didnt think the guy was completely on the ball but thought hey, id ask...

Ltdave wrote in post #14983011
started shooting film in 1976 with a Canon EXauto...

...............

no one TOLD ME this information, i BELIEVE it was a comment in one of the threads regarding the 17-55mm f2.8 IS USM lens...

There is a bit of misinformation found here from time to time and a there are things explained in ways that can be confusing.

First of all, the "crop factor" (called something different by various camera and lens manufacturers) was invented simply to allow someone familiar with using 35mm film format cameras to figure out what focal lengths to use on the then-new APS-C format DSLRs to achieve fields of view they were familiar with on their film cameras. Many folks who are relatively new to photography seem to have been led to believe that a 50mm lens magically becomes an 80mm lens when used on an APS-C format camera but this is totally untrue. The term "35mm equivalent" should be used more often than it is in discussions concerning "crop factor" issues

The bit you've read about the "equivalent" f-stops when comparing lenses used on so-called "full-frame" cameras with lenses used on APS-C format cameras is an attempt by some folks to involve depth of field calculations in their comparison. This is horribly silly in my opinion, as it gets some folks to believe - quite incorrectly - that there's an exposure-related difference between the two setups. You will get precisely the same exposure with the two different format cameras if you use the very same ISO, shutter speed, and f-stop settings on both cameras (regardless of the chosen focal lengths). There will very likely be some depth of field differences, though.

Is there anything specific that you may need help understanding? I'd be more than willing to help.

Bless you, Skip!

Just like there is a "35mm equivalent focal length" in mm, there is a "35mm equivalent DOF" in f-stop. Light transmission of a given f-stop is the same, be it a cell phone sensor or a MF camera sensor.

So the 24-105mm f/4 on full frame is the closest equivalent of the 17-55mm f/2.8 on crop in terms of equivalent focal length, DOF and sensitivity (given the lower high ISO noise of FF), but NOT in light transmission.

gotaudi wrote in post #14982733
I think you are referring to f/2.8 has the same DOF on a crop as f/4 does on a full frame. Light gathering is not the same f/2.8 will let more light in than f/4 on any camera. F stop is a ratio of the mechanics of a lens. The DOF, FOV changes from crop to full frame (assuming identical framing)

this is what the person was probably referring to...

using a longer focal length to retain the same framing:

on crop: 17-55IS @50mm 10ft away at f2.8= 1.29ft of DOF
on FF: 24-105mm @ 80mm 10ft away at f4= 1.12ft of DOF

both frame the same area, and as you see the 24-105L gives less DOF even though it's slower

or, moving closer to retain a similar framing:

on crop: 17-55IS @ 50mm 10ft away at f2.8=1.29ft of DOF
on FF: 24-105L @ 50mm 6.25ft away at f4= 1.12ft of DOF

of course before skip has to tell you the perspective will change in the second example because you are moving closer

so yeah in terms of light gathering the f4 is slower, but in terms of shallow DOF, the slower lens could certainly be considered equal to the 17-55IS

Ltdave wrote in post #14983011
i didnt think the guy was completely on the ball but thought hey, id ask...

Well IMO it's certainly not a silly question with all the FF vs. crop 'crap' out there.

If we were comparing it to a P&S I think there would be little issue, since few people are worried much about DOF on a P&S and don't try to compare focal lengths.

mannetti21 wrote in post #14983001
I don't understand why so many people try to draw up aperture equivalents between sensors. An aperture of f/2.8 is f/2.8 no matter what camera body it is on. The same amount of light enters no matter which way you try to cut it. The lens doesn't suddenly gain an improvement in light gathering if you take it off a 450D and put it on a 5D3.

Well, here is why. The way the camera works in making images is really affected by the size of the sensor. And when you have a really small sensor a lot of things (depth of field, low light capability) are just like having a much smaller aperture.

In a lot of ways, cameras with very small sensors like most P&S are great illustrations. A Panny LX5 (or whatever) may have a 5mm-30mm f/2.8 lens, but you are never going to take the same photo with that tiny sensor P&S as you would get with a larger sensor dSLR and standard zoom.

So I am one of those people that will tell you that having a 24-105L on a 5D Mark III body is very much like having a mythical 15-64 f/2.5 IS zoom on a body such as the 7D. Here is why.

Field of view is first and simplest. 24-105 on FF gives the same field of view that 15-64 gives on a 1.6X sensor.

DOF is second. Suppose I take a shot at 105mm and f/4 using the 24-105L on the 5D. To take the same photo with the same DOF with a 7D I need to shoot at 64mm focal length and f/2.5 aperture. Because I use a shorter focal length for any given framing and perspective with the smaller sensor, I need a larger aperture to arrive at the same DOF.

Finally, we have light gathering capability. Obviously, when it comes to transmission through the lens f/2.8 is f/2.8 the world over. But in general a FF camera of a certain generation will have less noise than a 1.6X camera of the same generation. The result is that I can use slower lenses on FF while compensating via higher ISO while not getting more noise.

So yes, the FF equivalent to the 17-55 IS is indeed the 24-105L, except it is wider, longer and effectively faster.

JeffreyG wrote in post #14983617
Well, here is why. The way the camera works in making images is really affected by the size of the sensor. And when you have a really small sensor a lot of things (depth of field, low light capability) are just like having a much smaller aperture.

In a lot of ways, cameras with very small sensors like most P&S are great illustrations. A Panny LX5 (or whatever) may have a 5mm-30mm f/2.8 lens, but you are never going to take the same photo with that tiny sensor P&S as you would get with a larger sensor dSLR and standard zoom.

So I am one of those people that will tell you that having a 24-105L on a 5D Mark III body is very much like having a mythical 15-64 f/2.5 IS zoom on a body such as the 7D. Here is why.

Field of view is first and simplest. 24-105 on FF gives the same field of view that 15-64 gives on a 1.6X sensor.

DOF is second. Suppose I take a shot at 105mm and f/4 using the 24-105L on the 5D. To take the same photo with the same DOF with a 7D I need to shoot at 64mm focal length and f/2.5 aperture. Because I use a shorter focal length for any given framing and perspective with the smaller sensor, I need a larger aperture to arrive at the same DOF.

Finally, we have light gathering capability. Obviously, when it comes to transmission through the lens f/2.8 is f/2.8 the world over. But in general a FF camera of a certain generation will have less noise than a 1.6X camera of the same generation. The result is that I can use slower lenses on FF while compensating via higher ISO while not getting more noise.

So yes, the FF equivalent to the 17-55 IS is indeed the 24-105L, except it is wider, longer and effectively faster.

No! The lens is not faster.. the ff body is...or can be.

DreDaze wrote in post #14983300
this is what the person was probably referring to...

using a longer focal length to retain the same framing:

on crop: 17-55IS @50mm 10ft away at f2.8= 1.29ft of DOF
on FF: 24-105mm @ 80mm 10ft away at f4= 1.12ft of DOF

both frame the same area, and as you see the 24-105L gives less DOF even though it's slower

or, moving closer to retain a similar framing:

on crop: 17-55IS @ 50mm 10ft away at f2.8=1.29ft of DOF
on FF: 24-105L @ 50mm 6.25ft away at f4= 1.12ft of DOF

of course before skip has to tell you the perspective will change in the second example because you are moving closer

so yeah in terms of light gathering the f4 is slower, but in terms of shallow DOF, the slower lens could certainly be considered equal to the 17-55IS

AND what happens when you are the same distance away at the same focal length setting at the same aperture setting?