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An Exercise in Perspective - and other things
Posted By: pacerr, 04-30-2011, 09:18 AM

In the beginning, back in the days when a pre-set Spiratone 400mm lens was just a dream, I had a crippled mindset that saw lens focal length only in terms of crop factor. e.g., telephoto to get closer, WA to widen the field of view.

After beating myself over the head and shoulders about composition it finally dawned on me that focal length was a way to control perspective rather than just crop factor - I could freely crop in the darkroom but I couldn't change perspective once the scene was captured on film. So I set myself an exercise to illustrate that concept and help form a different functional point of view. The phrase "zoom with your feet" has relevance only to the crop factor, not perspective, unless the FL of the lens changes as well.

Using wide, normal and telephoto lenses I'd pick a fairly open spot like a park or parking lot then at long, medium and close distances from a significant subject I'd explore the entire range of background perspectives and "vanishing points" at each focal length while concentrating on the change in perspective. I took notes and sketched diagrams of the specific changes at each position. That helped a lot in learning to better use lens focal lengths; and having to verbally describe the different views was also useful in its own way.

I also noted and recorded how the change in background areas affected exposure depending the tones included or excluded by the angle of view; that's something which seems to often be overlooked in questions regarding variances in exposure setting from lens to lens and with zooms or when changing metering modes.

At the time, I was interested in that topic as a means of recording "fidelity" in a forensic-type sense. I still find an occasional, abbreviated exercise useful to remind myself of the esthetic effects of focal length.

Today I use zoom lenses in that exercise for convenience. I also sometimes shoot each scene for later study, but back then that would have involved a significant expense in both darkroom time and film to record and categorize each profile. The ability to shoot digitally "for free" as a teaching/learning device today is a tremendous benefit that's often unappreciated.

As a trial, choose a mid-range zoom lens (the DA 18-55 kit lens for example) and shoot a simple object with a distinctive background at a distance of about four feet. Shoot one series at wide, normal and tele zoom settings from the same camera position and make a second series moving the camera to maintain the approximate subject size in the VF. Vary the aperture as well to the extent lighting allows to record changes in DoF.

Don't be too picky about exact procedures at first; you won't "get it right" the first time. The point is to study the method and revise the exercise to suit your personal needs in subsequent trials. Do practice verbally describing and sketching the the differences as you shoot though as this is both helpful and a valuable skill to acquire. There's no need to shoot high definition so the lowest JPG settings are fine and allow efficient archiving of the results if you wish.

EXIF data is useful to sort events and set up comparisons between sets. I find the COMPARISON mode in Faststone Image Viewer helpful in reviewing these (or any other type) comparison shots and I also use the included Screen Capture mode to facilitate storing and sharing specific examples.

Here's a very casual, 2-shot example from a Tamron AF 28-75. Re-sized only. Note the change in background and how multi-segment metering handled the difference in included values. A comprehensive series of about two dozen shots manipulating various combinations of exposures, object size, and camera position offers much practical insight.


This method of comparison can also be useful for recording and studying many different exercises besides FL perspective: comparison of metering modes, DoF vs. aperture, lens vs. lens, lighting set-ups, in-camera filter effects, etc.

H2
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04-30-2011, 06:26 PM   #2
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QuoteQuote:
The phrase "zoom with your feet" has relevance only to the crop factor, not perspective, unless the FL of the lens changes as well.
It's the other way round. The focal length only affects the crop but never the perspective. Changing the subject distance is what changes the perspective.

Cheers,
Tassilo
04-30-2011, 10:09 PM   #3
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QuoteOriginally posted by vparseval Quote
Changing the subject distance is what changes the perspective.
Unfortunately, I presumed an un-spoken variable regarding image size which may be confusing, but consider this:

Given a fixed FL lens, say 50mm, if I half the distance to the subject (zoom with my feet) the effect is the same as if I'd cropped the image an equivalent amount in printing it. The angle at which the lens "sees past" the subject relative to the background (perspective) hasn' t changed.

Imagine a fixed, conical beam defined by focal length projected ahead of the lens compared to another cone that's twice as wide but who's origin is only half the distance to the subject.

If I go from a 50mm to a 24mm lens and close the distance to keep the subject the same size, the subtended angles (perspective) change just as in my example.

One practical application is that a longer macro lens not only allows a greater standoff distance from the subject (crop factor), but it can potentially exclude much unwanted or distracting background behind the subject. Since the longer lens can reduce the area/proportion of out of focus background, it can also lessen the issue of "ugly" bokeh around the subject relative to a wider lens.

H2

This discussion would be an excellent excuse for someone with both the DA 35 and 100 Macro lenses to contribute a comparison based on perspective rather than sharpness.

Last edited by pacerr; 05-01-2011 at 06:03 AM.
05-01-2011, 06:35 AM   #4
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QuoteOriginally posted by vparseval Quote
It's the other way round. The focal length only affects the crop but never the perspective. Changing the subject distance is what changes the perspective.

Cheers,
Tassilo
It is the other way around, the original statement is correct, FL does change perspective not the distant to subject....

05-01-2011, 07:25 AM   #5
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Distance to the subject determines perspective. Here's a demonstration of that; I took these photos of a lantern through my office window. I changed only the focal length of the lens between shots.

Notice that when both lantern images are adjusted to be the same size you can clearly see the perspective is the same. The relative sizes of the lantern in the foreground and the garage in the background are the same.

As a beginner I argued that short lenses caused perspective change; I see now that is only true because you move closer to the subject to better fill the frame (it is the distance change, not the focal length, that makes the difference.)

Last edited by newarts; 05-02-2011 at 04:26 AM.
05-01-2011, 09:33 AM   #6
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newarts, did you change the height at which the camera was held between those two lens. I can clearly see more of the light pole above the window frame in the 55mm then in the 18mm.
It would also seem that I can see more of the top of the window frame in the 18mm then the 55mm but that could be because of the blurring in the 55 due to a difference in the DOF between the two.

Unfortunately I have to wait until tomorrow to try this myself.
05-01-2011, 09:53 AM   #7
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QuoteOriginally posted by ajlec2000 Quote
newarts, did you change the height at which the camera was held between those two lens. I can clearly see more of the light pole above the window frame in the 55mm then in the 18mm.
It would also seem that I can see more of the top of the window frame in the 18mm then the 55mm but that could be because of the blurring in the 55 due to a difference in the DOF between the two.

Unfortunately I have to wait until tomorrow to try this myself.
No I don't think so, I mainly just cropped them differently. I think the 18mm shot was cropped and enlarged while the 55mm shot was cropped and shrunk so that the lamp size on the display would come out the same.

It is a convincing demo I think. It sure convinced me.
05-01-2011, 04:36 PM   #8
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Okay, okay!

I apparently stand convicted of an improper definition, sloppy wordsmithing and a confusing and incomplete example. (But, hey, if you get to choose your own definitions you always win, right?) And the "logic", such as it is, has worked for my purposes for a long time so I've never actually tried to explain it before. Let me re-try it the Mickey Mouse way and y'all define it for yourselves.

It's the amount and appearance of the material present in the shaded area I'm interested in. The more I think about how I use this, the more I realize it's the awareness of how the included background will affect the choice of metering mode I use rather than 'composition' adjustments by FL. E.g, under what circumstances would I use TTL spot, CW, or matrix mode. Or possibly an incident metering mode. Then, of course, today there's always the lazy choice of just chimping til I get it right!

Move the lens position, angles of view, background depth and size the print/image to suit your own purposes.

At the risk of adding more confusion, I do agree that the "cropped" angle from the WA lens would have the same "properties" as the tele lens (green) but the "enlargement ratio" will be a factor if the subject is enlarged to the same viewing size.

If this causes anyone else to improvise their own exercises it's served a good purpose in any case. And thanks for challenging my sloppy start.

H2


Last edited by pacerr; 05-02-2011 at 08:04 AM.
05-02-2011, 04:18 AM   #9
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QuoteOriginally posted by pacerr Quote
Okay, okay!
It's the amount and appearance of the material present in the shaded area I'm interested in. ....

Move the lens position, angles of view, background dept and size the print/image to suit your own purposes.

At the risk of adding more confusion, I do agree that the "cropped" angle from the WA lens would have the same "properties" as the tele lens (green) but the "enlargement ratio" will be a factor if the subject is enlarged to the same viewing size......
Good illustration!

The example I gave of cropping & enlarging to show how perspective doesn't vary with focal length at a particular distance wasn't intended to argue that only one focal length lens need be in anybody's kit.

You correctly pointed out, ".. the 'enlargement ratio' will be a factor", in fact, my illustration was at about the limit of what image size manipulations are practical with cropping and enlarging.

Your comment about multi-point focusing being a factor when choosing focal lengths was enlightening for me as I only use center focusing and had not thought much about the influence of focal length except for subject isolation.

Here's an example with macros - the notice the difference in background for the two images which were taken along about the same line of sight with respect to the flower.


The perspective change hasn't altered the look of the silk flower noticeably but it sure has changed the over-all photo dramatically (I think that's a box of snack crackers in the background.... not the best choice of backgrounds

Last edited by newarts; 05-02-2011 at 04:23 AM.
05-02-2011, 08:02 AM   #10
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QuoteOriginally posted by newarts Quote
The perspective change hasn't altered the look of the silk flower noticeably but it sure has changed the over-all photo dramatically
Thanks for the useful background comparison. That's essentially the point I apparently wasn't verbalizing very well.

Did you happen to record and compare EXIF exposure data?

Referencing PhotoMe's EXIF data in the Manufacturer's Notes section one can observe an overwhelming amount of information about focus and exposure available to the camera logic. Even slight changes in the field of view, not to mention the user mode, could have a discrete impact on the focus and exposure results. Virtually none of that information is directly available in the view finder.

Another random consideration -- given that 'great bokeh' tends to homogenize the tones and the sharpness of both the foreground and the background, it must also have an averaging effect on overall contrast and EV and therefore have a direct impact on AWB, auto-focus and auto-exposure logic if the OoF areas are unintentionally allowed to affect those functions through in-attentive auto or manual mode selections.

H2

Whew, did all this really originate from a casual comment on FL selection vs. background presentation? Where'd I put my ol' Kodak Brownie?

Last edited by pacerr; 05-02-2011 at 08:56 AM.
05-02-2011, 08:35 AM   #11
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QuoteOriginally posted by pacerr Quote
Thanks for the useful comparison. That's essentially the point I apparently wasn't verbalizing very well.

Did you happen to record and compare EXIF exposure data?

H2
No I'm sorry no EXIF remains; I made no attempt to make good or accurate photos lacking much ability to do so. But I am certain the exposures, f-stops (f:4) - as stated on the posting - and ISOs were the same.

And you verbalized the point well. I was only adding a blunt force blow as a complement.

I've enough experience with my own abilities to know that many of us (esp me) need to be whacked upside the head a number of times before seeing what is obvious to others at first glance.

Dear wife told me it is a gender problem The most successful stage directors I know held discretion and understatement in disdain; she and he preferred a metaphorical baseball bat applied liberally and repeatedly. (Especially with beer drinking male audiences.)

Thirty odd years in front of classrooms filled with supposedly the best and brightest taught me the same. Like it or not, simply stating the truth once is insufficient to get people to buy in; alas, even truth must be sold like shampoo. Even to mathematicians.

Last edited by newarts; 05-02-2011 at 08:41 AM.
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