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Zoom lens vs prime lens - what's the disadvantage to zoom?

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Misty
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Zoom lens vs prime lens - what's the disadvantage to zoom?

Post by Misty » 11 Feb 2010, 11:23

This may or may not end up being relevant to me, since my scanner may be built around the Canon G10 I'm already using (and getting great results with). But that new 18mp Canon T2i/550D is looking awfully tempting to me, so if I can afford it I may end up with one of these.

That opens up the question of what lens to get, if I do end up going that route. I remember Dan mentioning before that he tries to shoot at the minimum zoom distance, which suggests to me that there may be some disadvantage to using zoom lenses. I'm not too familiar with zoom optics, so, Dan - what's the disadvantage? The other option would be using a prime lens. However, that also offers a disadvantage in some senses - using a zoom lens I can get higher resolution on smaller documents by zooming closer and minimizing wasted space around the document, and I would lose that on a prime lens.
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Re: Zoom lens vs prime lens - what's the disadvantage to zoom?

Post by spamsickle » 11 Feb 2010, 13:31

My setup was built around the minimum focusing distance for my cameras, about 21 inches. Even that close, I'm zooming in a bit to get more of the frame filled with the content I want. I don't go overboard, because there is some jitter from one frame to the next, and if I'm completely filling the frame on one shot there's a good chance that the next shot may cut off some of the content I want.

As long as you're using only optical zoom, I don't think there is a disadvantage that will be visible in the final product.

It might be interesting to do some experiments with moving the camera closer and shooting in macro mode to get the same sensor coverage, but I'd be surprised if that actually improved the final product.

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Re: Zoom lens vs prime lens - what's the disadvantage to zoom?

Post by IcantRead » 11 Feb 2010, 17:39

I have noticed with my setup that the closer I get the more reflection I get. It's mostly from the cameras being in the light. So I zoom about 2x optical. I don't knowtic any differace other than less reflection.

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Re: Zoom lens vs prime lens - what's the disadvantage to zoom?

Post by daniel_reetz » 11 Feb 2010, 23:55

The reason I initially shot (and recommended shooting) at the "widest" setting (on the A590s) was to compensate for two things:

1. Normal manufacturing tolerances -- the zooms don't necessarily zoom the same, but the way the lenses are built internally, the widest setting bumps against a bit of plastic, making it relatively reliable for matching two cameras.
2. It wasn't (and still isn't) clear to me that when using SDM to control the camera that "Zoom = 49" really means the same thing on both cameras. It's close, at least.

With a one camera setup, those differences are a lot less important.

About lenses on nice, big, expensive cameras:

Optically speaking, prime (fixed-focus) lenses tend to have less lens elements inside them. That is advantageous because each air/glass interface contributes a percent or two of glare (if it is Anti-Reflective or AR coated), or up to ten or twenty percent glare (non-coated). Almost no lenses are non-AR coated, so that really isn't an issue. However, if you have a complex zoom lens with 20 or 30 elements, even AR coated, that 1-2% of stray light reflecting off each air/glass interface and bouncing around the barrel of the lens really can reduce contrast and resolution. It's more of an issue if you are shooting outdoors, where a single intense source like the sun can cause lots of glare.

Zoom lenses, depending on their design, can also exhibit relatively complex spatial distortion.

I'm sure you're familiar with barrel distortion and pincusion distortion. They are sort of self-explaining. But the images you see of grids and stuff are idealized representations of distortion. Actually, most lenses, especially zoom lenses, can exhibit complex distortion which sort of "ripples" from the center and gets more extreme at the edges. It is often called "moustache distortion" which is more illustrative than the google image search for same.

With a prime lens, the distortion is a fixed thing, and usually minimal. The lens elements don't move much relative to each other, and the lens does not have a large retrofocus extension (necessary for SLRs, it's the back part of the lens moving fore and aft with focusing). Zoom lenses, on the contrary, will exhibit different distortion as they are zoomed/at different focal lengths. At the wide end, they typically exhibit barrel distortion, and at the far zoom, they typically exhibit pincushion or moustache distortion. In the middle, the image is relatively undistorted. The only issue there is that if you are concerned with correcting for that distortion, it simplifies things to have the focal length fixed.

There is one more issue, which is not very important but worth mentioning anyway. Some lenses for copy work are so-called "flat field" lenses. In non-flat field lenses, the focal plane is actually not a plane- it is somewhat curved. Flat field lenses attempt to keep the focal plane flat as can be. I have a 60mm Nikkor that is a pretty good attempt at a flat field lens. It's nice for shooting postal stamps and documents, but if your aperture is relatively smallish, this flat field or not thing really won't affect you much. Flat field lenses are usually most useful for very close-up work where the DOF is in mm, not inches or feet.

My suggestion to you, should you choose to go with the SLR, is to get a zoom lens. Even though the distortion is complex, and changing with focal length, using the EXIF reported focal length, it is possible to correct in an automated fashion using a utility like PTlens or Hugin. And I suspect for most projects you will be in the middle of the zoom range of your lens, so distortion may not be severe enough to be meaningful. The flexibility that a quality zoom lens will afford you will outweigh most imaging issues. At least with Nikon, some of the chromatic aberration and spatial distortion problems inherent in all lenses can be automatically corrected using their software (capture NX) at the time of RAW conversion. In other words, most of these kind of issues can be managed in software one way or the other.

Carl Zeiss has a nice paper that outlines the issues. You can more or less ignore the math and read their careful description and get a very good sense of the issues, though they are definitely not focused on document photography.

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Re: Zoom lens vs prime lens - what's the disadvantage to zoom?

Post by jonwilliamson » 12 Feb 2010, 20:29

In a nutshell, prime lenses are generally superior to zoom lenses because prime lenses usually give better quality/ sharper images. It really boils down to a tradeoff of flexibility vs. quality in most cases.

While some zooms are quite good, they are usually not as sharp as a comparable prime lens in the same range. While Daniel covers it from a more technical standpoint, I will give the answer that I normally give to non-photographers who ask the question.

Think of it this way - a prime lens can be optimized to give the very best quality for the one focal length it is designed for, but a zoom lens must be designed to give acceptable quality at all ranges of its zoom. As zoom ranges get larger, that usually means more and more compromises and potential problems such as diffraction, distortion, fringing, softness around the corners/light falloff, etc. Another thing to consider is that only the more expensive or top end zoom lenses offers constant aperture, so zooming in and out may change more than just the focal length.

For example, I would reference a couple of my favorite Canon lenses, the 50mm prime and the 24-70 mm L series zoom. Although being about 10 times heavier and 5 times more expensive, the 24-70L still doesn't yield images quite as sharp as the prime in most situations. But the zoom does give me more flexibility and range, so rather than change out prime lenses frequently I often just leave that on my camera. Now that is an extreme case, as it would require several Canon primes to cover the same range as the zoom, and the combined price would be much, much higher, but if I were looking at a fixed range that would not vary much then a prime might very well be a better option, especially if it were significantly cheaper.

But the bottom line is that like any other type of photography, you have to decide which one meets your specific needs the best.

FWIW, I plan to use a couple of primes on my scanner when it is finished, and just use physical adjustment to zoom in/out if needed. My thinking is that if I can get an acceptable DPI for the maximum scan size of my scanner, then I will just crop for smaller pages to maintain the same DPI.

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Re: Zoom lens vs prime lens - what's the disadvantage to zoom?

Post by Antoha-spb » 16 Feb 2010, 03:44

in my design i use canon 400d+tamron 17-50/2.8 and my experience (of some 40 books) pushes me to find a bright prime lens with manual focus (MF)

when a DSLR camera with zoom lens is looking upwards or downwards, the gravity pulls the 'front tube' outside or inside the rear tube (sorry i don't know the right names of the lens elements), as the mechanism isn't tight enough. With every shake of the scanner from platen movement and book [dis]placement the angle of view slightly changes so the camera needs to re-focus from time to time. And it's pity to see that one of 40 pages of a book that is already returned to library is shot with wrong focus.... Prime MF lens eliminate the 'unsanctioned shake-zooming' and the need to refocus saving the time.

As to frame targeting depending on page size - the letters in the books mostly don't depend upon the page size, so even if the camera targets the whole platen with a small-sized book over(under) it, the scans will still be good - just need to crop it. Therefore if cam's megapixels allow - one may use prime lens with no quality issues, imho.

A.

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Re: Zoom lens vs prime lens - what's the disadvantage to zoom?

Post by daniel_reetz » 16 Feb 2010, 08:50

Antoha-spb wrote:With every shake of the scanner from platen movement and book [dis]placement the angle of view slightly changes so the camera needs to re-focus from time to time.
Never would have thought of that. Experience is a hell of a teacher. I suppose if one engineered the scanner to be very stable this particular problem could be mitigated.

It's clear that we have the technical knowledge... but the fact that our forum members are actually building and trying these things really multiplies the value of that knowledge.

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Re: Zoom lens vs prime lens - what's the disadvantage to zoom?

Post by Misty » 16 Feb 2010, 11:57

Thanks a lot, Dan, Jon and Antona. That's an aspect I haven't thought before, Antona - that could be a big problem, I agree. It could make manual focus pretty impractical.

However, this is complicated by the fact that most of the items I'm going to be scanning are handwritten - there, the greater the resolution the better, which is why I've been hoping to be able to maximize available resolution for a given item's size. There's also likely to be a major variance in size - so far I've had to work with items from a quarter of A4 or less, up to maps 40 inches on either edge. Even given the quality advantages a prime lens's optics have that you bring up, Jon, I'm thinking that a zoom lens might be better at dealing with that kind of size variation in my specific case; however, prime lenses might have an advantage for people dealing with mostly similarly-sized items.
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Re: Zoom lens vs prime lens - what's the disadvantage to zoom?

Post by daniel_reetz » 16 Feb 2010, 13:55

One thing to consider is that Antona's fault may be one limited to lenses that extend out. Many designs do not extend the front elements outward. I know there's a word for that, but I don't know what it is.

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Re: Zoom lens vs prime lens - what's the disadvantage to zoom?

Post by spamsickle » 16 Feb 2010, 13:59

Antoha-spb wrote: it's pity to see that one of 40 pages of a book that is already returned to library is shot with wrong focus....
If only one page is shot with the wrong focus, and it isn't the last page, I'd suspect the cause was something other than camera shake causing the lens to droop. It isn't as though the lens is likely to re-align itself before the next shot, right?

This may or may not apply to your situation, but back when I was seeing this problem the real cause was that I wasn't getting manual focus at all. I was actually getting autofocus, and the stray pages that were out of focus were pages which didn't have text in the right place for the autofocus to work properly -- title pages, or pages at the end of a chapter, which only had a bit of text at the top of the page, and nothing in the middle where the autofocus was looking.

The Canons I'm using will drop out of manual focus and into auto focus mode if they go through a "sleep" cycle. That could happen if I paused in my scanning to answer the phone or make a sandwich. Now, my CHDK script makes sure that the cameras don't go to sleep at all, so it's no longer an issue for me. My cameras are attached to the platen itself, so they're probably shaking more than most people's cameras that are isolated from the moving parts by being off on a separate tripod.

My cameras output JPEGs, and my feeling is that that's a bigger source of image degradation than the fuzzy delta between a fixed and a zoom lens (not that I can change the lens on my cameras anyway). My final product is going to be the result of processing those images with Scan Tailor and a TIFF to PDF conversion tool, which will introduce further degradation. If I were shooting RAW images, and the images themselves were the final product (as in Misty's case), I might be concerned about my camera's optics, but since I'm not, I'm not.

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