Aspicus and the Art of Photography

From Snapper to Shooter (hopefully)

Project 1

Focal length and angle of view

What: The aim of this project is to explore focal length and angle of view, in particular the concept of standard focal length and its relationship to what can be seen with the eye.  The project involves taking one exposure at a focal length that aligns with what can be seen with the eye, one at a wide-angle and one zoomed in.  Finally the project involves taking a print of the shot at standard focal length and seeing at what distance from the eye it appears the same as the original scene.
Where: An alleyway very close to my house
When: Early morning in March. Weather was dry with some clouds in the sky.
Why: Given that the project calls for a print to be compared to the original scene, I wanted to take the images very close to my house. Additionally I wanted some sort of object in the frame that would make it easy to guage the differences between what the viewfinder could see and what my eye could see. Finally, the aim of the project was to demonstrate the idea of focal length, so I didn’t want to go overboard in the thought just yet. I decided that the alleyways behind my house would present the best opportunity and wandered around until I found the tree shown below.
How: Camera settings were manual, ISO200, f/11 and variable focal length, and somewhat surprisingly shutter speed (see below). I used a tripod as the full focal length (200mm) may have caused blurring if handheld (based on the rule of thumb of 1/focal length shutter speed for stable handheld shots).  No software manipulation other than the conversion from RAW to JPG.

The images:
Standard focal length (50mm marking on lens):

Focal length 50mm, 1/100sec shutter speed

Wide-angle (18mm marking on lens):

Focal length 18mm, 1/60sec shutter speed

Zoomed (200mm marking on lens):

Focal length 200mm, 1/125sec shutter speed

In terms of the images in relation to the scene, the 50mm actually seems somewhat zoomed in when compared to the scene.  See Observation 1 for a full description of why this might be so.

The final part of the project called for the student to print out the standard focal length shot, return to the scene, and hold the print-out in from of the scene and record the distance from the eyes at which point it aligned with the scene itself (i.e. appeared to be the same size).  I dutifully did this and came up with a length of approximately 50cm.  I’m not sure what this represents – a comfortably viewing distance?  This appears to be implied in the course notes.  Not sure what else to construe from it (I tried something similar with the other shots but unsurprisingly this didn’t really work).  I’ll give it some thought once I’ve fully digested my observations below.


Observation 1

I’ve known for a while that 50mm is considered standard, however I hadn’t realised that this was because it aligned with how we naturally see things, and (to me at least) it didn’t seem an approximation. 51mm and above and 49mm and below quickly diverged from my natural vision.

If you read my further questions below, one thing you will see is a brief discussion on how crop-factors fit into all this.  this seems tobe an area of much confusion looking around the internet.  Luckily some kind soul on the 18-200 lens Flickr group gave me an explanation.

What happens when you look through a 50mm focal length at a scene on a camera with (say) a 1.5 crop-factor?  Well the image is magnified in the viewfinder by a factor of 1.5 as you would expect.  However, the reason the image is magnified is because the field of view is decreased by a factor of 1.5 when compared to a 50mm lens on a full-frame camera.  This means that for a 50mm focal length, we see an image that is magnified by 1.5, but with a field of view of only 1.5 of our eyesight.  So the size of any objects in the image appears the same through the viewfinder (as we are seeing them magnified and shrunk by 1.5), but the image covers only a 1.5th of what we can actually see.

Now, when we adjust the focal length to 35mm, we expand the field of view by 50/35 ~ 1.5 times, so that it aligns with the field of view of our eyes.  Consequently the image is shrunk by a factor of 1.5 when compared to the 50mm case, and objects within the scene will appear as 1/1.5th the size of what we can see with our unaided eyes.

When we print out the images or view them on a monitor, we are no longer comparing the size of the images with what we can see and the field of view becomes the dominant factor.  So in fact the 50mm will seem zoomed in by a factor of 1.5 compared to what we could see (as we would looking at a window of 1/1.5th of what we could see) whereas we will see the entire scene we saw with our eyes in the 35mm focal length shot.

The upshot of all that, is that at 50mm focal length the size of objects in the viewfinder will the same as what we see with our naked eye, but it is the 35mm focal length image which will actually cover the same field of view of what we see once we print it out.

So looking at the 3 shots above, it is not surprising that the 50mm focal length shot seems zoomed in compared to what I remember seeing – it is, even though the elements of the scene would have appeared as the same size of what I could see through the cropped scene I was considering through the viewfinder.  If I had in fact taken a shot at 35mm then I would have had a scene that covered the same area as what I could see with the naked eye.

Observation 2

I was always a bit unsure of what happens at different focal ranges to the internal angles and relationships of an image i.e. is the image stretched out or transformed in any way.  Combining the images all together (see below) shows that there is no angular transformation (any apparent transformation in the image below  is through my dodgy compositing skills) and focal length is basically just a window into a scene.

Comparison of images at different focal ranges:

I thought I could use the above image to also look at what precisely the relative focal lengths also have a bearing on i.e. if we double the focal length – what changes – the area, the dimensions of the window, what? Measuring the dimensions of each of the above windows suggests that the relative focal lengths seem to work linearly for the 18mm and 50mm cases (i.e. the dimensions on the 50mm case are approximately 18/50 of the dimensions of the 18mm case) but not for the 200mm case. The dimensions of the 200mm case are no-one near 18/200 of the 18mm case (18/200 is around 0.09, whereas the dimensions of the 200mm case are 0.12 of the 18mm case). This may be of course that my measuring skills are not all they should be, or that I should be doing a basic arithmetic course rather than a photography one, but nevertheless it suggests (if correct) that these things do not necessarily work linearly, although they may do at the wide-angle to standard end of the scale. I also thought that area may be a factor but this isn’t so at all (no-where near in fact with the area of the 50mm case being around 7 time smaller than the 18mm, and the 200mm being 70 times smaller c.f. the factor of 0.09 above).

Further Questions

1. Why does the metered required exposure change at different focal lengths, even if the focal point does not move? If it is due to meter sensor size and the like then what are the implications of this? All the above shots were taken on fully manual, so that exposure what metered under the focal point. If, when zoomed out, there are more dark colours under the sensor’s footprint then this might mean that the shutter speed is adjusted accordingly.  I’ll try and get an answer from the web, OCA students, tutors and so on (or if you know please leave a comment).

2. DSLRs with APS-C sized sensors have a crop factor when compared to 35mm film cameras. For my Nikon D300 this crop factor is 1.5 – in other words, on a lens calibrated to a 35mm camera (or equivalent sized sensor), a focal length of 50mm will in fact be equivalent to 50 x 1.5 = 75mm. This is often thought of as a good thing in terms of telephoto zooms (and price), but not so much a good thing for wide-angled lenses or noise control. For this project, the thing that is confusing me slightly is that I was expecting the standard focal length (50mm on a 35mm equivalent camera) to also be subject to this crop factor, and the equivalent focal length on my setup to be 50/1.5 =33mm. I did take a shot at this focal length and it beared little relation in terms of size to what I could see without the camera. So why therefore does the 50mm still align with what I see without a camera given this crop factor? I’ll try and get an answer from the web, OCA students, tutors and so on (or if you know please leave a comment). (see Observations for reason)

3. What is the relationship between focal length and the dimensions of the scene. If I were to double the focal length, would I get a scene roughly half the dimensions? My rough sums above suggest maybe so at the smaller focal lengths, but probably not at the longer focal lengths. Hopefully a google search or two will sort this one out.

What I’ve Learned

1. Focal length basically means the photographer can shoot proportionally equivalent elements of the frame.  the higher the focal length, the closer into the frame the image becomes.

2. The crop factor of APS-C sensor cameras means that the focal length equivalent to eyesight is around 35mm in print, but 50mm through the viewfinder.


7 March 2009 - Posted by | Projects

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