sensor dictates 'actual' f-stop?

pyrojim wrote in post #14984206
AND what happens when you are the same distance away at the same focal length setting at the same aperture setting?

You have the same DOF, but the shots won't look the same...so that idea of a shot you have in your head will require you to do it differently on the two formats

But you know all that, so I don't know what you're trying to get at...

DreDaze wrote in post #14984254
You have the same DOF, but the shots won't look the same...so that idea of a shot you have in your head will require you to do it differently on the two formats

But you know all that, so I don't know what you're trying to get at...

Ok I'm guilty of fishing for that answer but at least it's out there now

There have been members here who swear the depth of field is different... And then they vary the parameters more than they understand they have!!!

WIth that headline "sensor dictates 'actual' f-stop" I thought about another topic - the microlens problem with the faster lenses. There the different sensors reduce the gathered light from fast lenses due to the angular responivity of the microlenses on the sensor.

mannetti21 wrote in post #14984211
LOL...Seriously though, I can't continue to debate this any longer.

So to you, the constant maximum aperture f/2.8 lenses offered on the Panasonic LX series compact cameras are exactly like having a 24-70/2.8 mounted on a 5D Mark III?

You don't see any need to account for the fact that these two cameras with the same 'effective' focal length and same aperture will not be able to make the same image in most situations?

For that matter, why is an effective focal length OK but an effective aperture is misleading? You'd tell someone that 17-55 on crop is like 28-90 on FF, but f/2.8 is always the same? It isn't.

pyrojim wrote in post #14984470
There have been members here who swear the depth of field is different... And then they vary the parameters more than they understand they have!!!

The depth of field is different, and the only variable you should be changing is the focal length.

If you want to talk about taking the same photo with two formats (which is the only thing that makes sense for us to be discussing) then the cameras must be in the same place.

Then the larger the format, the longer the focal length we have to use to get the same field of view. The result will be less DOF with the larger format unless we also use a smaller aperture.

It's not that complicated except that people insist on doing two misleading things when discussing it:
1) Then talk about moving the camera, which means it is certainly not the same shot.
2) Or they talk about shooting from the same spot with the same focal length. Again, this is not the same picture as the larger format will have a much larger field of view. And if we crop the larger format, then it isn't a larger format anymore....it is the same format.

pyrojim wrote in post #14984470
Ok I'm guilty of fishing for that answer but at least it's out there now

There have been members here who swear the depth of field is different... And then they vary the parameters more than they understand they have!!!

The one variable that you didn't discuss was final magnification or print size. Assuming that both images would undergo the exact magnification (which would yield a smaller final image with your crop sensor) then the dof would remain the same. The confusion comes into play when you are comparing two identical final print sizes from two different sensor sizes because that requires two different magnification factors from each sensor....obviously, the larger sensor would undergo less magnification. When calculating DOF, the magnification of the final image must be taken into consideration......it'​s a very important variable that can't be overlooked (an effective DOF can't be calculated without knowing the final print size).
So, in your scenario, although the crop sensor would yield an identical image of the center crop of your FF sensor, if you magnify it a greater amount to compare two prints at the same size, your DOF will be slightly different. On the other hand, if you magnify it the exact same amount as your FF image (yielding a smaller overall print with the crop sensor), the dof would be identical.

Conventionally, the "magnification" to final image size that Jason refers to is called "enlargement" as when using film, the device traditionally used to project the image onto paper is called an enlarger.

In the case of the crop image at same FL, aperture and shooting position, enlarged to the same size, the crop image has 1/1.6 or 62.5% the DoF of the FF shot, although it also has only the central 62.5% of the image. This is exactly the same as the difference between crop and FF when the focal length is adjusted to achieve the equivalent field of view, except in that case, FF's DoF is shallower. 1.6x results in about one and a third f-stops DoF (1.4x would be exactly one stop).

I know exactly what thread/conversation you're referring to, what wasn't really discussed in depth was that while DoF will indeed be affected by using a larger medium, in this case full frame vs. aps-c, the light gathering capabilities remain the same. f/2.8 = f/2.8 regardless of format. The argument that a full frame sensor makes your lens "faster" kind of works, but only in the sense that a sensor with larger photosites is able to gather more light data, so you have more usable ISO range than a smaller sensor with smaller photosites. Of course, at equivalent ISOs, the light gathering or speed should be exactly the same. If you have a larger sensor with an ultra-high resolution, and hence relatively small photosites, the same argument won't work.

JeffreyG wrote in post #14984975
So to you, the constant maximum aperture f/2.8 lenses offered on the Panasonic LX series compact cameras are exactly like having a 24-70/2.8 mounted on a 5D Mark III?

You don't see any need to account for the fact that these two cameras with the same 'effective' focal length and same aperture will not be able toake the same image in most situations?

For that matter, why is an effective focal length OK but an effective aperture is misleading? You'd tell someone that 17-55 on crop is like 28-90 on FF, but f/2.8 is always the same? It isn't.

What I said was in regards to the amount of light coming through the lens, nothing else. So in that case, yes, 2.8 on a point and shoot would be the same as 2.8 on a full frame, as would be the same as 2.8 aperture in front of my eyeball. And yes, effective aperture is indeed misleading because of the variance in sensor technology even within the same format, thus making the comparison useless unless you specify exact camera models for each discussion. So basically, what happens to the light after it goes through the aperture is dependent on the sensor, not the aperture.

The reason effective focal length comparison is NOT misleading is because the crop factor is a quantitative value of 1.6x, and if people are talking about 1.3x cameras, they will specify. Again, I never commented on 2.8 being globally equivalent, in terms of light AND dof.

I believe we are possibly both correct in our understanding, but perhaps our semantics disagree. I guess my overall point is simply that for an individual who is completely new to photography, I can see how they could be easily confused into thinking that, in terms of light gathering, an f4 lens will magically become f2.8 as soon as they attach it to any FF sensor.

JeffreyG wrote in post #14984975
For that matter, why is an effective focal length OK but an effective aperture is misleading? You'd tell someone that 17-55 on crop is like 28-90 on FF, but f/2.8 is always the same? It isn't.

mannetti21 wrote in post #14985218
The reason effective focal length comparison is NOT misleading is because the crop factor is a quantitative value of 1.6x, and if people are talking about 1.3x cameras, they will specify. Again, I never commented on 2.8 being globally equivalent, in terms of light AND dof.

In fact, there is absolutely no such thing as an "effective focal length". That term suggests that there is some camera format which is the end-all standard against which all other camera formats in the world are measured against and that simply is not so.

The 35mm film format (24mm by 36mm) is not, never was, and never will be such a universal standard. It is simply the format of the cameras that are the approximate physical size that today's DSLRs were modeled after so that the existing lenses for the 35mm film cameras could be used on them.

Thus, using the terms "effective focal length" and "effective aperture" are totally meaningless unless the comparison details are explained along with the terms.

Further, the "effective aperture" thing is quite silly in my opinion, because that term does not indicate that the only factor being considered in the "effective" part is depth of field. There is absolutely no difference in exposure settings when a lens is moved from one format camera to another.

So, moral of the story, the 24-105 is NOT a FF equivalent to the 17-55 lens. The only equivalent lens would, in fact, be another 17-55 f/2.8. And in regards to the thread title, the sensor does NOT dictate the amount of light travelling through the aperture...what happens beyond the rear end of the lens should be a completely different discussion.

mannetti21 wrote in post #14985480
So, moral of the story, the 24-105 is NOT a FF equivalent to the 17-55 lens. The only equivalent lens would, in fact, be another 17-55 f/2.8. And in regards to the thread title, the sensor does NOT dictate the amount of light travelling through the aperture...what happens beyond the rear end of the lens should be a completely different discussion.

So for you, the equivalent of a wide to short tele zoom on a 1.6x camera would be an ultra-wide to standard zoom on a FF camera?

1Tanker wrote in post #14983816
No! The lens is not faster.. the ff body is...or can be.

Yes and important to note. Don't confuse the terms of the technology--a "cow" is not a "bull" for people who have to work with cattle.

It's the capabilties of the body that allow the use of a higher ISO to provide the same image quality...and different bodies even in the same format have different capabilities. An f/2.8 lens on a 5D1 does not suddenly become an f/1.0 lens on a 5D3 just because the 5D3 has better image quality at a higher ISO.

Unless we're talking about T-stops.

And I'm probably already sorry I brought that up.