AOP Project 1b. Exercise: Focal length and angle of view

Find the standard focal length for my camera and understand how it relates to how far away from my eye the resulting A4 picture needs to be.

Exercise: Following the instructions, I pointed my camera at a scene, and matched up the view through the viewfinder with the view through my other eye.  This is ‘standard’.  I took photos of the standard focal length, and then for each lens I had, I took a photo of the widest and furthest telephoto settings – and noted the focal length on the lens.  (The photo file also records the focal length.)

I then printed all the photos onto A4 paper and went back to the same spot and held the ‘standard’ print in front of me and repositioned myself until the printed scene was the same as the view in the real scene. See image 1.  I then noted the distance between the photo and my eye.  I did this for some of the other photos.

Image 1 – Image 1: Demonstration of linking a standard focal length A4 print to the view seen by the eye.

Personal learning:  I went into this exercise just following the instructions and didn’t have any idea as to what the outcome was going to be, however, when I started to take the first photos I stated to think if there was some correlation between the focal length and distance to my eye.  I was quite excited to get home and try the rest of the exercise.  As soon as I printed the standard shot I had already decided that there had to be a direct link between the focal length number and the distance from my eye.  Rain, lack of daylight and work got in the way for a few days, but at the first opportunity I went out and checked the distance and was really pleased to see I was right.  Once I understood the concept with the first couple of prints I started to guess the correct distance for the next two and then checked it.  Some of the telephoto prints I just deduced the results from the concept.  Some of the reading are off by a mm or two but I think this is due to the fact that the distance is up to the sensor location and not my eye.  Thus the importance of the focal plane mark on the camera as per my previous blog.

When I took the shots I expected that the standard would be 50mm but it turned out to be 52mm.  I’m not sure if this was just me, or user error.  I’ve seen the section in my D90 user manual on ‘calculating picture angle’ which directly relates to this, so I’m going to review that again.   I also have some questions around how this exercise works with a full-sized sensor.  My D90 DX sensor is a bit smaller than the standard APS-C  and I do know that I need to consider the ‘magnification factor’ caused by the sensor size (100mm is actually 150mm, therefore magnification factor is 1.5).  And I was wondering why A4 sized paper?  I the USA, the standard paper size is ‘letter’, a bit longer and narrower than A4 – how would this exercise impact on them?

I am really pleased to understand the link between distance from the focal point to the lens length – one technique I had been taught a while ago is to look though a frame to identify the shot to be taken.  My initial frustration was trying to get the right frame to the size of print (understanding 3×2 has now helped with this!) but I never consciously thought about telephoto distance.  This is going to be very useful to help me identify the right focal length and therefore correct lens to use for the type of photo I want to take.

Outstanding questions:

• Am I right in saying the standard for me is 52mm and not 50?  Was it user error or typical?
• Because of D90 DX sensor is a bit smaller than the standard APS-C I know that I need to consider the ‘magnification factor’ caused by the sensor size (100mm is actually 150mm, therefore magnification factor is 1.5).  How does this impact this exercise? What if my sensor was full-sized?
• My camera is a 3:2 aspect ratio – how would this work for camera with a different aspect ratio?
• Why A4 paper size?  North America use a different standard (letter) of print material, how will this exercise impact on this?

Results are as follows:

COMMENT	FOCAL LENGTH	DISTANCE FROM EYE WHEN PRINTED ON A4
Sigma 10:20mm	10mm	11cm
Nikon DX 18-105	18mm	19cm
Sigma 10:20mm	20mm	21cm (deduced)
Standard	52mm	53cm
Nikon 70:300	70mm	71cm
Nikon DX 18-105	105mm	106cm (deduced)
Nikon 70:300	300mm	301 cm (deduced)