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02/23/2006 05:08:37 PM · #1 |
I learned this from a professional portrait photographer yesterday. To determine the correct focal length to use with YOUR camera to take portraits you first have to know your sensor size, look in the manual. Determine the length and width of the sensor then square each number. Add the two results and then find the square root of that number. Whatever that number is double it and that will give you the correct focal length to use with your camera for portraits.
Example: My sensor’s dimensions are 22.7 X 15.1
22.7 squared = 515.29
15.1 squared = 228.01
515.29 + 228.01 = 743.30
The square root of 743.3 = 27.26
Double 27.26 = 54.52
Therefore a focal length of 55 would be ideal for taking portraits with MY camera.
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02/23/2006 05:18:58 PM · #2 |
| Thanks for the info....I'll check mine when I get home tonight. |
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02/23/2006 05:30:18 PM · #3 |
56.74 is what i get for the D70s
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02/23/2006 05:31:10 PM · #4 |
| is that before or after the crop? |
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02/23/2006 05:38:58 PM · #5 |
| Once again the 50mm f/1.8 shows what a dynamo it is for friggin $75... |
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02/23/2006 05:45:07 PM · #6 |
| hmm...but with the 1.6 factor the 50 is really an 80. Still a nice lense though. I see it is fairly popular. |
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02/23/2006 05:45:56 PM · #7 |
are you trying to get 85mm on FF equivalent? Cause you're at 87mm there. I don't get all the math really but 68mm would be what my camera would need to act like an 85mm.
But the ideal lens length depends on the effect you want. |
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02/23/2006 05:56:34 PM · #8 |
| the 50mm f/1.8 is probably gonna be my next glass purchase! Lovely lens. |
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02/23/2006 06:00:03 PM · #9 |
The 50mm f/1.8 is a fun lens but I don't like it for portraits. I find I have to be much too close to get what I want. I suppose it would be okay for full length portraits, though.
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02/23/2006 06:00:15 PM · #10 |
Originally posted by PhotoRyno:
I learned this from a professional portrait photographer yesterday. To determine the correct focal length to use with YOUR camera to take portraits you first have to know your sensor size, look in the manual. Determine the length and width of the sensor then square each number. Add the two results and then find the square root of that number. Whatever that number is double it and that will give you the correct focal length to use with your camera for portraits.
Example: My sensor’s dimensions are 22.7 X 15.1
22.7 squared = 515.29
15.1 squared = 228.01
515.29 + 228.01 = 743.30
The square root of 743.3 = 27.26
Double 27.26 = 54.52
Therefore a focal length of 55 would be ideal for taking portraits with MY camera. |
Cool, you did the math for me ;-)
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02/23/2006 06:06:44 PM · #11 |
| Perhaps I'm missing something but why is this math useful? Can you just look through the view finder and a take a step forward or back to get the crop you want (assuming a prime lens)? I get the idea of knowing what focus length you need for stuff where you might need to act fast but why would you need it for a studio portrait shot? |
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02/23/2006 06:21:02 PM · #12 |
I don't understand why such a rule ?
What Kyebosh said makes sense and possibly a reason..
In that case forget all the maths and just find your focal length by diving 87mm by your crop factor...
1.6x crop means 87/1.6 = 54.4
1.5x crop means 87/1.5 = 56
1.3x crop means 87/1.3 = 66.9
Message edited by author 2006-02-23 18:21:23. |
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02/23/2006 06:21:39 PM · #13 |
I don't get it.
The right lens depends on the working distance to subject.
I prefer a 50mm f1.4 or my 77mm f1.8 ltd but the choice is made by how much room I have to work in.
bazz. |
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02/23/2006 06:22:27 PM · #14 |
Originally posted by sir_bazz:
I don't get it.
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I don't either :) |
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02/23/2006 06:23:43 PM · #15 |
I agree with bazz...the right lens depends on many factors...
working distance to the subject, lighting equipment available, size of your studio, depth of field that you want, etc, etc...
saying that there is one right choice for any camera for taking portraits is utter hogwash.
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02/23/2006 06:29:13 PM · #16 |
That is cool, interesting isnt it when someone like a professional photographer with a successful business doing probably what they love is kind enough to tell you these bits of facts.. Specially the math (I hate math) Math is all in photography too! You got to consider other things as well light, speed, aperature setting, check this out...
Math used in photography
I have met some photographers in our group here that gave a good demonstration when they opened their studio about light and portraits..
Math was the biggest topic to understand and when you or if you purchase light gear you will want to know what space coverage voltage and such before you set things up. Keep learning it doesnt hurt :)
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02/23/2006 06:31:18 PM · #17 |
| The best lens is the lens that gives the desired result. To many rules. |
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02/23/2006 06:34:36 PM · #18 |
Originally posted by iamkmaniam:
The best lens is the lens that gives the desired result. To many rules. |
Yup, I agree with all the great lenses, whatever looks good is the perfect match. Now the Sigma Bigma may look good at 100 feet and if that's where you have to be then it's perfect too ;) |
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02/23/2006 06:43:42 PM · #19 |
If I have to do that kind of math just to take a picture, I think I'll go ahead and eBay my kit right now. ;)
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02/23/2006 06:46:23 PM · #20 |
Originally posted by PhantomEWO:
Originally posted by iamkmaniam:
The best lens is the lens that gives the desired result. To many rules. |
Yup, I agree with all the great lenses, whatever looks good is the perfect match. Now the Sigma Bigma may look good at 100 feet and if that's where you have to be then it's perfect too ;) |
Well, with the bigma at 100 feet, you'd end up with a really flat portrait, but sometimes that's what's wanted. Lens choice is directly correlated to Desired effect.
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02/23/2006 06:47:22 PM · #21 |
i'll jsut keep using the 50mm 1.8
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02/23/2006 07:13:20 PM · #22 |
Does anyone else find it interesting that this is an equation for a circle?
The above reduces to
(f/2)^2 = L^2 + W^2
f = focal length
L = Length of sensor
W = Width of sensor
I'd like to see who derived this and why, ie- the physics of it. I've been playing with equations but have yet to find anything relevant.
Yes, I need a life. |
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02/23/2006 07:28:00 PM · #23 |
Most general theory says 80-100mm, perhaps 120, on 35mm film, so 50-85 or so on a 1.5/1.6 crop camera sould be about right. But it all depends on you, your style. What does the pro say the perfect aperture is? Perfect shutter speed?
I like this - a 70-210 at 146mm (233mm 35mm equivalent)
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02/23/2006 07:42:15 PM · #24 |
Basically, it's just double the length of the diagonal (the formula described is using Pythagoras' theorem to calculate the diagonal, and then doubling it). Equivalently, this will be close to double the size of the "normal" lens for the format/sensor in use.
For comparison, a 35mm film sensor gives 86.5mm, roughly double the "normal" lens for 35mm, a 50mm lens. An APS-C sensor gives 60mm (this would apply to most digital cameras). Similarly, for 6x6 medium format, 170mm, roughly double the "normal" lens for 6x6 which is 80mm.
If you boil it all down, what it's really saying is that for portraits, the best lens is a mild telephoto. Which many of us knew or intuitively discovered anyway.
I have an honours degree in mathematics, but I would not attempt to argue that the level of precision described is warranted. Is a 90mm lens really that much better than a 100mm lens for portraits (assuming use of a 35mm camera)? Using a digital camera, will the difference between using 55mm, 60mm or 70mm be such that everyone instantly picks the one taken at 60mm as the best? I don't think so. At best, I'd take it as a general guideline, a rule of thumb, not something to be applied religiously.
It seems like some peoples' obsession with using the rule of thirds, calculating it down to the millimetre. Illustrative story: At my friend's camera club, one of the judges is known for being obsessed with the rule of thirds and ignoring any sense of artistry. Once when he was scheduled to be judging, my friend submitted a photo of her cat; the photo was uninspired but she'd cropped it and measured it so that the cat's eyes were exactly at the rule of thirds intersection. The judge's comment was "This photo carries no emotion for me, but the rule of thirds was applied perfectly - I give it 13 out of 15." This confirmed my friend's complete lack of respect for the judge.
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02/23/2006 07:46:12 PM · #25 |
The math is simply the calculation of the sensor diagonal (the distance from corner to corner of the sensor), multiplied by two. For a FF sensor, the calculation yields: sqrt(24^2+36^2)43.26mm*2 = 86.5mm, or about what you'd expect for a "traditional" portrait focal length on a 35mm camera. What you're doing, then, when calculating it for a different sensor size is just calculating the focal length that will have an "85mm-ish" FoV on that size sensor. BTW, it works with ANY sensor or film size, from Point & shoot to large format.
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