Beginner Focal Lengths

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I thought this was the best place to post this, as it's about equipment, rather than in the Beginner's section.

Anyway, I'm still "getting into" photography, and have now had my DSLR almost a year.

Just looking at various lenses, and then a question popped into my head - What does focal length translate to?

By that, what I mean, if a lens goes up to 400mm, what does that mean in "real figures"? IE, what distance can you shoot an object from? I know there are other factors such as tripod, SS etc etc, but let's say a bird sat on a branch, how close would you have to be, or how far away could you sit to get a really good picture?

Is there some sort of easy formula to give me a better understanding?
 
The easiest way to understand it is to draw a triangle.

Start with a vertical line either 36mm high or 24mm high (full frame or cropped sensor longest dimension respectively) then draw a horizontal line from the middle of your vertical line, the same length as the lenses focal length.

Next, join up the three ends of the lines to form a triangle. That gives you the angle of view for the longest side of the frame.

This is easy if you have a CAD drawing programme as you can measure the angle but paper and pencil is good too. You can then scale this angle up to see how far away you need to be to fill a frame with an object.

angle1.JPG


This is a 110mm focal length with a 24mm wide (cropped) sensor giving an angle of view of 12 degree.

Scale the mm up to feet and if you had a subject 24 feet wide, you would need to be 110 feet away to fill the frame with it.


Steve.
 
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Oh I do like a bit of maths on a Wednesday morning. That makes more sense.

Would I be right in thinking my camera is 24mm? (Canon EOS1100D?)
 
Yes. The 1100D has what is known as an APS sized sensor - based on the dimensions of APS film. The sensor size is 24mm x 18mm so if you want to know the angle of view to fill the short (normally verical) side then substitute 18 for 24 in the triangle.

Another way of thinking about it is that if you divide the focal length by the frame size, you will get a factor which when multiplied by the width (or height) of the subject will tell you how far away you need to be.

e.g. Taking a full length shot of a six foot tall person rotating the camera 90 degrees to portrait mode using a 70mm lens. Divide 70 by 24 (long length of sensor - now vertical) = 2.91. Multiply the persons height by this number - 6 x 2.91 = 17.5

so with a 70mm lens and a 6' tall person, you would need to be 17.5' away to fill the frame with this person's image. Obviously this puts the top of his head at the top edge and his toes at the bottom. You would need to move back a bit to get some space around the subject.


Steve.
 
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:) yes that will have a cropped sensor.

In practice different lenses have different minimum focal distances too (MFD) - some just won't focus if you are too close to your subject.
 
Steve Smith said:
Yes. The 1100D has what is known as an APS sized sensor - based on the dimensions of APS film. The sensor size is 24mm x 18mm so if you want to know the angle of view to fill the short (normally verical) side then substitute 18 for 24 in the triangle.

Another way of thinking about it is that if you divide the focal length by the frame size, you will get a factor which when multiplied by the width (or height) of the subject will tell you how far away you need to be.

e.g. Taking a full length shot of a six foot tall person rotating the camera 90 degrees to portrait mode using a 70mm lens. Divide 70 by 24 (long length of sensor - now vertical) = 2.91. Multiply the persons height by this number - 6 x 2.91 = 17.5

so with a 70mm lens and a 6' tall person, you would need to be 17.5' away to fill the frame with this person's image. Obviously this puts the top of his head at the top edge and his toes at the bottom. You would need to move back a bit to get some space around the subject.


Steve.
Click to expand...

Now that is easier maths that drawing triangles :LOL:
 
Steve Smith said:
Yes. The 1100D has what is known as an APS sized sensor - based on the dimensions of APS film. The sensor size is 24mm x 18mm so if you want to know the angle of view to fill the short (normally verical) side then substitute 18 for 24 in the triangle..
Click to expand...

You're a fair bit out with your sensor dimensions, Steve. A Canon APS-C sensor is 3x2 aspect ratio and 24x18 is 4x3 aspect ratio.

The 1100D sensor is 22.2 x 14.8 mm

Bob
 
Other ways:

Use a focal length comparator widget, like this one from Tamron http://www.lencarta.com/studio-lighting-shopfront/non-folding-softboxes/softbox-honeycomb-grids

Or an online calculator like this (scroll down to Angular Field of View Calculator) http://www.tawbaware.com/maxlyons/calc.htm

Or, take your camera and lens, set to 50mm. Frame up a subject the same size as you want to photograph, eg for a small garden bird, use a mug or similar ornament. Note the distance. Now if you use a 100mm lens, you'd get the same framing from double the distance, with a 200mm lens double the distance again, with a 400mm lens double again and so on. Short answer, if it's brids you're after, is a very long lens and then get as close as you can ;)
 
Canon Bob said:
The 1100D sensor is 22.2 x 14.8 mm
Click to expand...

In that case, it's not APS-C size!

Just put the actual dimension in and the maths will still work.

EDIT: Looks like there are as many standards as there are manufacturers. In theory, it should be a full 35mm film frame of 36 x 24 cut in half. i.e. 18 x 24.


Steve.
 
Steve Smith said:
....... In theory, it should be a full 35mm film frame of 36 x 24 cut in half. i.e. 18 x 24.
Click to expand...
Your maths is still wrong Steve. 36x24 is 3:2 aspect ratio but 24x18 is 4:3....you're changing the shape when you're reducing the size.

Bob
 
Steve Smith said:
In that case, it's not APS-C size!

Just put the actual dimension in and the maths will still work.

EDIT: Looks like there are as many standards as there are manufacturers. In theory, it should be a full 35mm film frame of 36 x 24 cut in half. i.e. 18 x 24.


Steve.
Click to expand...

APS-C was not half-frame (though close to it) and no 'APS-C' DSLR has ever been properly APS-C format either (though close to it - 25.1x16.7mm). http://en.wikipedia.org/wiki/Advanced_Photo_System
 
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