Tutorial How big a lens do I need?

StewartR

StewartR

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How big a lens do I need? - How big a lens do I need?

(Mods: I'm hoping this might be suitable material for the Tutorials section.)

In my line of business I often get people asking how big a lens (ie what focal length) they'll need for a specfic task. It's actually quite easy to work out if you know - or can estimate - how big the subject is, and how far away it will be. Here's how.

The basic equation is:

Focal Length = 0.75 x Sensor Size x Subject Distance / Subject Size

Here are some examples. In each case I'm...
Click to expand...

Read more about this resource...
 
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sorry to be a bit anal, but is the end result the actual focal length on the lens, or is it the equivalent 35mm focal length....
 
(y) This equation is now programmed into my PDA.
Cheers.
 
For those who are more interested in the mathematics.

To compute field of view across one edge of a sensor:
2*arctan(frame size/(focal length*2*(m+1)))

Where m is the magnification caused by the focusing of the lens. At infinity m=0. This is why lenses change focal length as you focus them (unless corrected in the lens by other elements/gears.)

From there, it's quite simple to work out the dimensional field of view. If you understand the equation above, I'm sure you understand how to work out the dimensions of a triangle, so I'll leave it to the reader.

If you're going to put it in your PDA you may as well put the actual equation in, not just an approximation.

Edit:
The equation given by Stewart works well due to a certain approximation that's being used. It's perfectly good for doing things in your head, or where a few degrees here and there don't make a difference (especially as you're normally working with estimates anyway), but if you're going to do it on a calculator you may as well do it correctly.
 
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It's not the hubble telescope :) I think an approximation would do just fine(y)
 
That's really useful Stewart. Thanks :)

For mental loafers like me, how about 20 x distance / size?

I guess you could also turn it round, which would help people that want to photograph birds in the garden, given that the lens is usually fixed to a max of, say, 300mm. How close do you need to get?
 
StewartR said:
(Mods: I'm hoping this might be suitable material for the Tutorials section.)
Click to expand...

I'm sure there must be plenty more useful stuff like this hidden round the forums.

I won't move this to tutorials just yet but i will soon.

If anyone spots more hidden gems can you please report the post
report.gif
and just say in the report 'possible tutorial' or something similar.
 
Wow thread from the dead hoppy, but a bloody useful one (and has answered a q in my mind atm)
rearranges to:
distance= focal length * subject size / 0.75 * sensor size

can also be rearranged to (losing the constant as I'm talking proportionality but no proportional sign on kboard):
distance/focal length = subject size/sensor size
which makes a lot of logical sense

I'll shove it into excel mobile :D(and one day code an app for it)
 
HoppyUK said:
For mental loafers like me, how about 20 x distance / size?
Click to expand...
Well, the short dimension of a crop sensor is about 15mm and the long dimension of a full frame sensor is 36mm, so you have a potential error factor of a bit more than 2 with your approach. If you're happy with that, fine.
HoppyUK said:
I guess you could also turn it round, which would help people that want to photograph birds in the garden, given that the lens is usually fixed to a max of, say, 300mm. How close do you need to get?
Click to expand...
Sure. It looks like this:

Subject Distance = Focal Length * Subject Size * 1.33 / Sensor Size

The 1.33 is the arbitrary factor to aid composition; it's the reciprocal of the factor of 0.75 used previously. If you use the equation this way round, remember to measure the subject size and the subject distance in the same units! (Focal length and sensor size are both measured in mm.)

Here's an example - again, using my Canon 350D which has a sensor measuring 22.2 x 14.8mm.

Suppose we want to photograph a goldfinch, like this, with your 300mm lens.

339607192_zDTVS-M.jpg


The goldfinch is about 15cm long - which we'll express as 0.15m because metres are handier units for determining the subject distance. The sensor dimension which matters here is the 14.8mm because we're shooting the goldfinch in landscape orientation - the height of the bird aligns with the short dimension of the sensor.

So the distance we need to get is 300 * 0.15 * 1.33 / 14.8 = 4 metres.

(In practice that might be a bit difficult to achieve without a hide. I shot this photo from our lounge at a range of about 20 metres with f=2000mm.)
 
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Where did you find that squirrel...needs shooting, not photographing.
 
Yep i get it... why the 0.75 bit though?
 
StewartR said:
Hope this helps. By the way, the experts amongst you will have spotted that the factor of 0.75 in the equation is an arbitrary one to help give you a decent composition - you don't want your giraffe / squirrel / moon to completely fill the frame, and this factor is there to give your subject bit of room to breathe in the frame.
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Lynton said:
Yep i get it... why the 0.75 bit though?
Click to expand...

Last paragraph of Stewarts opening post (y)
 
Stewart / Martyn,

Apologies - one day i will learn to read the entire post before fingers hit keyboard, before eyes and brain are switched on........ there was me thinking last night at a wedding reception hmmm 4/3rds, well the reciprocal of that is 3/4 or 0.75 so that will be why.........:LOL:
 
My god I must be so stupid.... I'm gonna quit while I'm behind.... Think I'll be needing a new hobby now.
 
Is the same equation meant to work for calculating what kind of wide angle lens you need?

I was in a potential wedding venue on Sunday that was fairly small. I only had my D80 with a 50mm lens with me but I stood at the back of the room and took a photo of where the bride would be and only got her top half in.

The calculations are these:

0.75 * 23.6mm (in Portrait Mode) * 10m (front to back of room) / 1.7m (approx height of bride)

According to that I could have the bride fill 3/4 of the frame with a 105mm lens. So how come my 50mm lens could only get half of her in so I was assuming I'd need about a 20mm lens to get her all in.

Am I being daft and calculating it all wrong?
 
10m is about 30ft and 1.7m is about 5ft6.

I've done it in metric and imperial and come out with the same answer that with a 23.6mm sensor in portrait you can get a head to toe portrait of a person of average height from 30ft (10m) away with about a 100mm lens yet I can only get half of them in with a 50mm lens.
 
-Rob- said:
10m is about 30ft and 1.7m is about 5ft6.

I've done it in metric and imperial and come out with the same answer that with a 23.6mm sensor in portrait you can get a head to toe portrait of a person of average height from 30ft (10m) away with about a 100mm lens yet I can only get half of them in with a 50mm lens.
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You're doing something wrong, Rob.

I just tested it. Our office is exactly 10m long and my willing assistant is about 1m65. I got her to stand with her back to the wall whilst I framed her with a 70-200mm zoom on a Nikon D70.

She filled the frame, hear to toe, top to bottom, at f=135mm. I then adjusted it to give me a better composition (ie a bit of space around her) and I found I was around f=105mm.

So my practical experience agrees with the maths.
 
easylife said:
Still work in old money...how many feet is that.. :thinking::thinking::thinking:
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As loing as you measure your sensor and your focal length in millimetres, which is quite natural, then you can use any old units for the subject size and distance. Feet, yards, rods, poles, perches, light years, whatever. Just remember to use the same units for each measurement!
 
Sorry but this is totally useless information when taking pictures of wildlife. You don't have the time to sit down and work out the maths involved.

Realspeed
 
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I think the point is that the (very useful) information above would allow you to decide which lenses you needed before you went and stood in the field waiting for the owl.
 
Lord help me,

I'm new to all this photography stuff, but it looks like I need to be spending money on a few lenses to cover what I intend on capturing. I just hope my loving wife likes the end results.

Dave
 
I've been directed here by Suvv, very handy explainations/calculations/ transpositions, cheers. (y)

So would that work out if you photographed something 15cm tall 20m away with 400mm lens with 22.3mm sensor, you would only fill about 0.13 of the frame with the subject?
 
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Andy- said:
I've been directed here by Suvv, very handy explainations/calculations/ transpositions, cheers. (y)

So would that work out if you photographed something 15cm tall 20m away with 400mm lens with 22.3mm sensor, you would only fill about 0.13 of the frame with the subject?
Click to expand...

You can do it another way.

Frame up a similar sized subject with whatever lens, and note the focal length and distance. If you double the distance, you must double the focal length to maintain the same size image.

For exaample, if it's right at 1m with a 100mm lens, then it will stay the same size at 2m with 200mm, 4m with 400mm and so on, doubling each time.

You will quickly discover that subjects about 15cm tall, like small garden birds, need a very long lens from a much closer distance than you might hope.

There's another problem with small subjects too. The marked focal length on most zooms is fairly loose, and at close distance the internal workings actually reduce the focal length still fruther, sometimes quite markedly. (In the specs, focal length and aperture are measured at infinity.) It's not unusal for a 70-300mm zoom to be more like an effective 250mm at the long end, when focused at 2-3m distance. Primes are better in this respect.
 
I wonder if anyone has done an iphone app or online calculator to work it out??
 
Cheers Richard,
I posted in “Talk Basics - Lens and distance”; folks have been helpful.
Just working out an appropriate lens and compromise for what I'll be using the camera for, what can be expected and achieved, and make sure there’ll be no disappointments or over-expectations.
All the tips and advice is very useful. Great site! :)
Regards, Andy
 
Years ago we used to teach the 'slide-rule' method - hold an empty 35mm slide in front of you, move it back and forth to get the framing you want of the subject as your looking through the slide and the distance the slide is from your eye (in millimeters) is the approx focal length of the lens you need. For crop sensors you would need to modify the cardboard slide holder slightly so the opening matched your sensor size.
It really helped people new to photography understand focal length AND framing/composition.

Randy
 
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