Depth of field: Full-frame lens vs. APS-C lens, both on APS-C sensor

[runnah]

I am getting a strong sense of déjà vu.

Does that mean that this thread is gonna "make my dick itch?"
I went through the thread I recommended and see that Brianeack was involved in that thread trying to discuss this topic with WayneF.

I got a cream.

 

[Derrel]

Cameraclicker: Are you aware that depth of field in the close-up range is basically independent of lens focal length, and is instead dependent pretty much on MAGNIFICATION of the subject matter? Yes, it's true.

You mention how things ought to be able to be measured....but as I mentioned above in my reference to the Bob Atkins article on Depth of field, depth of field behavior can be categorized into three zones: 1) the CLOSE-UP zone 2)Intermediate distances and 3)Longer distances.

Again, as I mentioned earlier, depth of field behavior can be broken down into three different "types" of situations/shooting conditions. Your example of the three focusing targets demonstrate one of the anomalies that I mentioned above, some 70 posts back, that in the close-up range, DOF is dependent upon MAGNIFICATION of the subject, and lens focal length becomes basically....totally irrelevant. Weird, but true.

The reason your three-card focus chart shows basically the SAME depth of field is that it is a CLOSE-RANGE shot, and depth of field is caused because the three subjects have been reproduced at the same magnification.

Maybe look into the science of it. Here's what I mean.

Close-up
When the subject distance

approaches the lens focal length, the focal length no longer is negligible, and the approximate formulas above cannot be used without introducing significant error. At close distances, the hyperfocal distance has little applicability, and it usually is more convenient to express DOF in terms of magnification. The distance is small in comparison with the hyperfocal distance, so the simplified formula

can be used with good accuracy. For a given magnification, DOF is independent of focal length."

emphasis mine:For a given magnification, DOF is independent of focal length. Your shot of the three cards pretty much PROVES the theory, which you seem to have been unaware of...


from Depth of field - Wikipedia, the free encyclopedia

As you can see, Depth of Field is broken down under section 10 and 11 into sub-sections dealing with the different ways DOF behaves in close-up, moderate, and longer distance scenarios.

 

[runnah]

I don't think we'll ever really know how this all works. It's a mystery, like dark matter, string theory and how planes fly.

 

[Derrel]

The Online Photographer blog devoted about one week, several articles, and hundreds of comments and questions to exploring the different factors behind the science of depth of field. It's NOT that difficult to understand, but there are so many people spouting B.S. that it is difficult to know what is truth, partial truth, and utter B.S..

Like the way at reallllllly close distances, such as on close-up photos, the focal length no longer really affects DOF--but the magnification is what determines the DOF. That is why, on those three focus target cards, shot with different formats, with different lenses, from close range, but shown at the SAME degree of magnification, the DOF is independent of the focal length. The same behavior does NOT hold true once the distances grow longer.

Maybe we could think of it like a 500-pound bomb; if one goes off in a living room, the entire room is destroyed and EVERYBODY in there DIES...buuuut....75 yards away, NOT everybody dies, and the measurable performance of the bomb's blast changes significantly...and at a LONG distance, nothing is felt, just a little bit of noise is heard. Farther away still, only the smoke plume is seen. So: how do we characterize the explosion of a 500 pound bomb? Do we go with the close-up description? The intermediate? The long range? Or from, basically "Infinity"?

This thread started off with a simple question, but got filled up with different questions, misunderstandings, poor writing, replies that were messed up or misdirected, yadda yadda...we have people who have theory but no actual experience telling us they "KNOW" the science, yet seem to be really, actually, UN-aware of the science, to a pretty high level.

 

[runnah]

i have yet to see a simple article/video describing how it all works. I have seen many that contradict each other. So honestly I have given up trying to fully understand the science behind it. I know how to use is practically and that is fine with me.

 

[Overread]

The problem is much as Derrel points out - the physics behind it is solid and fixed; BUT a lot of people don't understand it - or only understand part of it. As a result when they teach others the second hand info is already incomplete. It then gets repeated out - mixed up - jumbled up and in the end we end up with a big mess of cross conflicting information.

It's partly the result of the fact that most of us are users of photography, but not scientists understanding its full function). There are also some guidelines that sound like fact mixed in there too.


Take Derrels' example of macro photography. Most macro shooters will understand it; however even within those circles people get confused. The background blurring on short focal length lenses is far less than on long - a 35mm can appear to have more depth of field than a 300mm macro lens on the same shot at the same magnification - because whilst the depth of field itself is the same the blurring of the background is far less distinct (esp if the shot has the subject grading into the background instead of a sharp drop of and then a longer distance to the background subject content).

 

[CameraClicker]

Cameraclicker: Are you aware that depth of field in the close-up range is basically independent of lens focal length, and is instead dependent pretty much on MAGNIFICATION of the subject matter? Yes, it's true.

You mention how things ought to be able to be measured....but as I mentioned above in my reference to the Bob Atkins article on Depth of field, depth of field behavior can be categorized into three zones: 1) the CLOSE-UP zone 2)Intermediate distances and 3)Longer distances.

Again, as I mentioned earlier, depth of field behavior can be broken down into three different "types" of situations/shooting conditions. Your example of the three focusing targets demonstrate one of the anomalies that I mentioned above, some 70 posts back, that in the close-up range, DOF is dependent upon MAGNIFICATION of the subject, and lens focal length becomes basically....totally irrelevant. Weird, but true.

The reason your three-card focus chart shows basically the SAME depth of field is that it is a CLOSE-RANGE shot, and depth of field is caused because the three subjects have been reproduced at the same magnification.

Maybe look into the science of it. Here's what I mean.

Close-up
When the subject distance

approaches the lens focal length, the focal length no longer is negligible, and the approximate formulas above cannot be used without introducing significant error. At close distances, the hyperfocal distance has little applicability, and it usually is more convenient to express DOF in terms of magnification. The distance is small in comparison with the hyperfocal distance, so the simplified formula

can be used with good accuracy. For a given magnification, DOF is independent of focal length."

emphasis mine:For a given magnification, DOF is independent of focal length. Your shot of the three cards pretty much PROVES the theory, which you seem to have been unaware of...


from Depth of field - Wikipedia, the free encyclopedia

As you can see, Depth of Field is broken down under section 10 and 11 into sub-sections dealing with the different ways DOF behaves in close-up, moderate, and longer distance scenarios.

Last time I did this, I used a shorter lens and longer distance and got a similar result. I didn't think the distance was all that close, and don't see a huge difference when looking at the whole files. I thought shooting new files with a longer lens would be faster than looking for the old files and chose this lens because the 150 has it's own tripod mount. I'm out of time again. I probably won't get back to it until next week. Sounds like you recommend I do my own calculations using the formula instead of relying on the calculator.

 

[photoguy99]

****in magnets. How do they work?

 

[shefjr]

"How exactly is a rainbow made? How exactly does the sunset? How exactly does the posi-trac rear end on a Plymouth work? It just does!"
Joe Dirts dad

 

[Ysarex]

Joe, you may have to quote your niece again. LOL!

Let me run this by you.
Here are DOF calculator results for what I did:
View attachment 87479

So on the calculator it says "Use the actual focal length of the lens". So I use some APS-C body to take a picture. Then I use a full frame body, from the same place, same ISO, same aperture, same lens. The calculator says APS-C should be total depth of 0.09 ft, or 1.08 inches. It says the full frame should have more DOF, 0.14 ft, or 1.68 inches -- 56% more!

When I take those photos and look at the results, View attachment 87480 , I don't see 56% more! A couple of people here have said they see less, maybe, with full frame.

In my world, mathematical modeling is supposed to produce a result that you can get out a ruler and measure. The experiment is not showing what the calculator seems to be predicting.

In the side by side DOF Master frames you're comparing two different photos taken with two different format cameras. In order to make a meaningful comparison you have to take the same photo with the two different cameras. In this example you need a 95mm lens on the 7D for the two photos to be the same. That will give you more DOF from the 7D and a hyperfocal distance of 550 feet instead of 870 feet -- as expected.

Joe

 

[photoguy99]

I just skimmed the thread but it doesn't look like anyone had pointed out that it's all a fool's game.

If you don't know how you're going to crop and what the final presentation size is and how far away viewers are going to be from the final photo, you can't calculate DOF as such. Also, Dof is a perceptual thing so even there one viewer is going to see it differently than another.

The online calculators can tell you what will most like be sharp enough, so you can tell if the model's ear and nose will both be sharp enough to count as in focus. Or if the tree and hill will be.

Generally.

If you print big, or if your audience is going to be nose to nose with the photo, you need to be more conservative still.

What they won't do is a good job of telling you what's not 'in focus'. Mostly we look at photos smaller than the conservative model assumes so there's more DOF than the model suggests. More stuff is sharp, in focus, than dofmaster said would be.

Open up for less DOF. Chimp to see if you got it. It's crude and dumb, but it's just about at effective as mucking about with calculators and formulae.

 

[Ysarex]

I just skimmed the thread but it doesn't look like anyone had pointed out that it's all a fool's game.

If you don't know how you're going to crop and what the final presentation size is and how far away viewers are going to be from the final photo, you can't calculate DOF as such. Also, Dof is a perceptual thing so even there one viewer is going to see it differently than another.

The online calculators can tell you what will most like be sharp enough, so you can tell if the model's ear and nose will both be sharp enough to count as in focus. Or if the tree and hill will be.

Generally.

If you print big, or if your audience is going to be nose to nose with the photo, you need to be more conservative still.

What they won't do is a good job of telling you what's not 'in focus'. Mostly we look at photos smaller than the conservative model assumes so there's more DOF than the model suggests. More stuff is sharp, in focus, than dofmaster said would be.

Open up for less DOF. Chimp to see if you got it. It's crude and dumb, but it's just about at effective as mucking about with calculators and formulae.

You did just skim. There were a lot of tangents and divergent sub posts but also one consistent question. There's a suggestion in post 11 and an assertion in post 13 that sensor size has no effect on DOF. In fact it does and it's taken a rather long time and some effort to get to the correct answer.

Complex as it may be DOF is a photographic phenomena that is understandable and definable and successfully manipulable within a reasonable working range. Just because it's complex doesn't mean we should relegate it to the province of fools.

Joe

 

[photoguy99]

Sure you can understand it and manipulate it. If you don't know your final print size when you're shooting, though, it's all wildly approximate.

Dofmaster and friends, with their answers out to 1/100th of a foot or whatever, yield a rather odd impression.

By the by, when I say it's a fool's game, I don't mean to call anyone a fool. It's an idiomatic phrase meaning roughly a pointless exercise.

 

[Ysarex]

Sure you can understand it and manipulate it. If you don't know your final print size when you're shooting, though, it's all wildly approximate.

No. It is slightly approximate and so works well. For example it's empirically established that people will view a print at an average distance relative to the size of the print. For example when's the last time you viewed a highway billboard from a distance of 6 feet? And even if you did we could discount you as anomaly. Averages have been calculated and they apply.

Yes there's no need for three digit precision in the calculators, but the math behind the calculations is well tested and functional.

Joe

Dofmaster and friends, with their answers out to 1/100th of a foot or whatever, yield a rather odd impression.

By the by, when I say it's a fool's game, I don't mean to call anyone a fool. It's an idiomatic phrase meaning roughly a pointless exercise.

 

[photoguy99]

I sit a pretty constant distance from my computer screen. Guess where I look at almost all photos?

I think you place excessive faith in these things, but I am ok with just leaving it as a disagreement.

ETA: the various calculators and formulae do indeed work well to ensure that stuff we want to be in focus is in fact sufficiently sharp. Because they are fairly conservative. What they are less useful for, and largely nobody cares, is telling us what's out of focus

They don't really tell us DOF. They give us a moderately strong and useful lower bound.