Focal length and shooting distance
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HoppyUK
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I don't think you can generalise, like zooms do and primes don't, and they vary a lot. I tested some 70-200 2.8s recently and the Nikon was quite bad at this, under 150mm at closest distance. Canon and Sigma not so much, and Tamron (cheapest) was least affected.
I have a few macro lenses here now, all primes of course, and I reckon most of the 100mm-ish ones are under 80mm at 1:1. It's a design decision, and any internal focusing lens (ie just about everything these days) can do it if the designer feels like it.
On the other hand, I'd be very surprised if something like a Canon or Nikon super-prime lost focal length to any significant degree, though I've not checked any.
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Roy C
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Thanks for that 'Hoppy'. The reason why I asked was because I have seen numerous test between long lenses like the Canon 400/5.6 prime v Canon 100-400, Sigma 50-500 and Sigma 150-500 comparing the focal lengths at 400mm when shot from typical bird photography distances like 20-30 ft. In every test the 400 prime was significantly longer than the zooms. I was also reading the other day about someone who compared the Sigma 100-300 v a Sigma 300mm prime, the conclusion being that the zoom got nowhere near the focal length of the prime when shot at anything less than infinity. Could it be that telephoto zooms or more likely to lose focal length than long primes?
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Roy,
It seems that manufacturers don't supply this info in a usable format. They do however give us the information to work it out.
Given the maximum magnification figure and the minimum focus distance, it becomes a little exercise in trigonometry.
For a full frame body;
(( .036 x 1/Max mag)/2) / MFD = Inv tan / 2 of the horizontal AOV
Bob
It seems that manufacturers don't supply this info in a usable format. They do however give us the information to work it out.
Given the maximum magnification figure and the minimum focus distance, it becomes a little exercise in trigonometry.
For a full frame body;
(( .036 x 1/Max mag)/2) / MFD = Inv tan / 2 of the horizontal AOV
Bob
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Roy C
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A bit late for me to re learn trig Bob but I believe you
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Roy C
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The reason why I was asking is because I have often doubted that my SX40 'superzoom' gets anywhere near the stated focal lengths when shooting at typical distances for small birds.
To this end I done a quick test which compared the SX40 v DSLR with prime lens.
The DSLR combo was the 40D + 1.4x tc + a 600mm astro telscope which should work out a full frame equivalent FOV of 1344mm (600 x 1.4 x 1.6).
The SX40 was at full zoom with the 2x tc activated which should be a full frame equivalent of 1680mm (840 x 2).
Both shots were taken from 22 feet. As can be seen, not only is the superzoom not as 'long' as the DSLR combo but it is considerably shorter.
#1 DSLR combo should be 1344mm
#2 SX40 should be 1680mm
To this end I done a quick test which compared the SX40 v DSLR with prime lens.
The DSLR combo was the 40D + 1.4x tc + a 600mm astro telscope which should work out a full frame equivalent FOV of 1344mm (600 x 1.4 x 1.6).
The SX40 was at full zoom with the 2x tc activated which should be a full frame equivalent of 1680mm (840 x 2).
Both shots were taken from 22 feet. As can be seen, not only is the superzoom not as 'long' as the DSLR combo but it is considerably shorter.
#1 DSLR combo should be 1344mm
#2 SX40 should be 1680mm
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I suspect that your reasoning for this statement is wrong, Roy.
A parallel explanation might be easier.
Stick a 100mm macro lens on your camera and focus to MFD. As you rightly stated in an earlier post, the focal length (due to the design) will be around 78mm, and the AOV will roughly support that dimension.
This 78mm focal length is fixed and is an optical property of the lens. Now put a 50mm extension tube between the lens and the camera. The lens is still at MFD and is still 78mm FL but the image on the sensor is larger....it's been cropped by moving the sensor further from the exit pupil and gives the impression of a narrower field of view (which would lead you to believe that the focal length has increased.
Make sense?
Bob
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Roy C
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Yep it makes sense Bob but I did say FOV (and not focal length). All I am trying to do is to try and work out a correlation between a 'Superzoom' and a DSLR as 'reach' is everything when it comes to bird photography. I know many people who have bought a superzoom solely because they think they will be getting a lot more focal length then their DSLR set-up, In most cases and when shooting at typical bird shooting distances I reckon they will be very dissapointed!
Lets put it another way Bob, Canon say the the SX40 is 840mm full frame equivalent at the long end (their words not mine) - what size lens would you need on a DSLR 1.6 cropper to equal the FOV (or perceived reach) of the superzoom. I would calculate it as 525mm (840/1.6). This may be to simplistic but how else can you get an idea?
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I appreciate that, Roy, but you've arrived at your "projected FoV" by using focal length...which isn't possible with the setup you describe.
I'll ponder the rest and get back later.
Bob
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I understand your difficulty Roy, and I'm not sure there are any easy or reliable answers. Manufacturers often don't quote the data you need to make calculations, perhaps because they don't want you to know, and when it comes to very close shooting, like macro, the difference between the minimum focusing distance (measured from the focal plane/sensor, as quoted by manufacturers) and the one that matters, the minimum working distance (measured from the front of the lens, also used for DoF calcs) can be significantly different.
Traditionally we've always used focal length as the shorthand for field-of-view/angle-of-view and magnification, so the way I do it is to sit two lenses side by side and focused at a relevant distance, then measure the field-of-view and convert that to angle-of-view with a bit of basic trig. Then go to an on-line calculator and convert that to focal length.
From what I know though, it is clear that there are a few general trends that I think are grounds to raise questions, if not accurate answers. Super-zooms are one, like 18-200mm and more. The designers there are under great pressure to make them as compact as possible while still offering good close focusing. Given the choice of extending the lens in the conventional way, or shifting a few elements around inside to get the same effect (even at the expense of focal length) it's obvious which way is favourite.
Same goes for longer zooms, 100-400mm and 50-500mm etc, when the amount of physical extension necessary is much greater and combined with zooming demands as well, this imposes mechcanical difficulties as well as making the lens impractically long.
Without hard data, your opening assumption is probably not far off - if you want maximum reach at close range with a long lens, for small garden birds perhaps, then go for a prime. It's not a given, but I suspect there won't be too many exceptions.
Traditionally we've always used focal length as the shorthand for field-of-view/angle-of-view and magnification, so the way I do it is to sit two lenses side by side and focused at a relevant distance, then measure the field-of-view and convert that to angle-of-view with a bit of basic trig. Then go to an on-line calculator and convert that to focal length.
From what I know though, it is clear that there are a few general trends that I think are grounds to raise questions, if not accurate answers. Super-zooms are one, like 18-200mm and more. The designers there are under great pressure to make them as compact as possible while still offering good close focusing. Given the choice of extending the lens in the conventional way, or shifting a few elements around inside to get the same effect (even at the expense of focal length) it's obvious which way is favourite.
Same goes for longer zooms, 100-400mm and 50-500mm etc, when the amount of physical extension necessary is much greater and combined with zooming demands as well, this imposes mechcanical difficulties as well as making the lens impractically long.
Without hard data, your opening assumption is probably not far off - if you want maximum reach at close range with a long lens, for small garden birds perhaps, then go for a prime. It's not a given, but I suspect there won't be too many exceptions.
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Thanks for your feedback guys .
I know of several people who own lenses like the 400/5.6 and shoot with a 1.6 cropper DSLR and think that they can gain a lot of reach by getting a superzoom like the SX40 (supposedly 840mm full frame equivalent). I guess the message I was trying to get across was that for shooting the likes of birds from a typical distance (15-30 feet) these superzooms are not going to give you anywhere near the reach you may be led to believe!!
.I know of several people who own lenses like the 400/5.6 and shoot with a 1.6 cropper DSLR and think that they can gain a lot of reach by getting a superzoom like the SX40 (supposedly 840mm full frame equivalent). I guess the message I was trying to get across was that for shooting the likes of birds from a typical distance (15-30 feet) these superzooms are not going to give you anywhere near the reach you may be led to believe!!