LENS FOCAL LENGTH TEST

I was interested to see how much difference there was between lenses of different focal lengths, and to see if using Teleconverters had an impact on focus & frame sharpness. So I decided to shoot the same object using the same center of focus (top of a power pole) that was some distance away, with each lens and compare the prints..

Pictures were taken over the course of a few days, so actual scene content may change
(such as shadows, grass, clouds, even loss of tree leaves)


Focal Length LENS USED F-Stop Shutter Speed
35 mm SMC Takumar 1:3.5 / 35mm F-16 1 / 250
50 mm SMC Takumar 1:4 / 50mm F-22 1 / 125
55 mm SMC Takumar 1:2 / 55mm F-16 1 / 250
85 mm Eyemik 1:4 / 70 - 220mm Zoom F-11 1 / 500
105 mm Eyemik 1:4 / 70 - 220mm Zoom F-11 1 / 500
135 mm Eyemik 1:4 / 70 - 220mm Zoom F-11 1 / 500
220 mm Eyemik 1:4 / 70 - 220mm Zoom F-22 1 / 60
300 mm Hanimex 1:5.5 / 300mm F-32 1 / 15
440 mm Eyemik 1:4 / 70 - 220mm Zoom, with 2X Teleconverter F-22 1 / 15
600 mm Hanimex 1:5.5 / 300mm, with 2X Teleconverter F-5.5 1 / 125


Click on button for image you wish to see
(none are preloaded and each image is from 120kb to 150kb in size (950 x 631 resolution) )


LENS FOCAL LENGTH iMAGE COMPARiSON
                     


Now there are some interesting things to note. (They may not be interesting to you if you have an automatic camera and you never do any adjustments yourself (such as f-stop and shutter speed), but my old Pentax Spotmatic II camera is a manual only camera, so those things are important to me.


iSSUE #1 DEPTH OF FiELD
You probably already know that f-stop plays a vital role in determining the depth of field (as does object focal distance), but what is interesting, is IF you set your lens to infinity focus, then f-stop has no impact, on your subject in relation to all objects further away. I tested this, with the 600mm focal length, I took a shot at F-32 with 1/2 speed shutter, then shot the same object at F-5.5, with 1/125 speed shutter. Both pictures had the same depth of field, because for both shots the focus of the lens was at infinity.

So to make taking pictures easier, I've found that when your subject is at infinity distance, use the lowest f-stop (widest aperture) your lens has. This then allows you to use the fastest shutter speed possible (with the available light), which reduces the problems of camera shake (especially at long focal lengths) or moving objects (ie trees moving in the wind) and DOES NOT shorten your depth of field in reference to your main object and beyond.

Try this yourself. Find a subject that is just at or just before infinity focus for whichever lens you use (preferably have objects further behind your focal object). Select your widest aperture (say F-2 or F-4) and focus your subject. Now notice the focus of all background objects aswell, then increase the F-stop on your lens to your smallest aperture (say F-22). What did you notice with depth of field?.. Well you should have noticed nothing. But now if you do that same test with a closer object (say only 1 - 5 meters away) you will indeed see that reducing the aperture increases the depth of field. Of course if you have close objects in the frame (closer than your subject, which is currently at infinity), then reducing the aperture (increasing f-stop) can help bring them into focus.


iSSUE #2 TELECONVERTERS
It seems that the teleconverter does not impact much on clarity of the image, when using infinity focus. All images appear to exhibit no distortion or lack of clarity. The Teleconverter, does however impact on your f-stop, and a 2X converter adds 2 f-stops to your base lens maximum aperture..


iSSUE #3 PERSPECTiVE and FORESHORTENiNG DiSTORTiON
It is interesting to note that the wide angle lens (the 35mm) portrays the hills (which are quite a distance away (over 10km's) as being a very long distance away. However using the 600mm focal length, this has made the hills look extremely close (maybe only 1km away). So the longer your focal length, the more foreshortening distortion occurs. This can have a benefit though. If you take a picture of something you wish to portray as being longer (such as a city street, or hallway), then use a wide angle lens. If you wish the object to look shorter, then use a telephoto or Zoom lens.

This is why you should NEVER use a wide angle lens for portrait photography because it will distort the perception of the persons face between the difference in distance (reference to the camera) of their nose to their ears. Using a lens above normal focal lengths (say 85mm to 135mm) will foreshorten the features of the face, so the nose and ears will look correct and in proportion.

An interesting tip I read in a book ( The Asahi Pentax Way ), is that you can use a wide angle lens to help remove unwanted objects from the background. Say you have an object that is directly behind your subject (when using a 55mm lens). If you now use a wide angle lens (say 35mm) the subject is smaller, so you now move in closer so the subject is the same size as it appeared in the viewfinder with the 55mm lens. Now you will notice that the offending background object has been pushed off to the distance considerably futher, and most of the time, it's enough to remove it from being directly behind your subject. Of course don't get too close to your subject with the wide angle else you will have issues with perspective distortion, but it's a handy tip to remember: If you can't move the subject to a better location, use a wide angle lens to move the offending background objects out of the way :) See the next section of this page for an example of this. Keep an eye on the black tree near the center of the frame, at 55mm it's pushed further away, but at 600mm it appears to be right at the steps, directly behind the rose bush in the green pot.


( Above images were scanned in using a Epson Perfection 2480 PHOTO flatbed scanner (scanning the print, not the negative) at 600 DPI, then bicubic resized down to 917 x 600 resolution to make viewing easier in a 1024 x 768 desktop (stored as JPEG). )



LENS iMAGE QUALiTY COMPARiSON

A similar test to the one above, EXCEPT this time I am attempting to use the same object size, but using different focal lengths and distance to the subject. This test is to see if there is any frame detail loss due to focal lengths, and to see if using Teleconverters have any impact of frame detail and sharpness.

I have chosen a different subject this time, and even though the picture appears very "busy", this was done on purpose because I wanted a scene with multiple objects all with different textures to help determine any differences between focal length and object distance. All images taken with camera on Manfrotto Fluidhead Tripod with cable release.

I have tried to shoot the same frame composition for each of the 3 images, but as you can imagine it's quite difficult to get the exact frame composition when at 3 different distances from the object, but hopefully you will still be able to see what I'm trying to accomplish.

Each image was shot on the same day (within a 10 minute period) at the same camera settings. F-11 Shutter 1/250, using Agfa ASA200 film.

Focal Length LENS USED F-Stop Shutter Speed
55 mm SMC Takumar 1:2 / 55mm F-11 1 / 250
300 mm Hanimex 1:5.5 / 300mm F-11 1 / 250
600 mm Hanimex 1:5.5 / 300mm, with 2X Teleconverter F-11 1 / 250


Click on button for image you wish to see
(none are preloaded and each image is from 160kb to 200kb in size (896 x 600 resolution) )


LENS FOCAL LENGTH iMAGE QUALiTY COMPARiSON versus OBJECT DiSTANCE
       



iSSUE #1 DEPTH OF FiELD
Depth of field now is quite crucial because I'm not at infinity focus you can clearly see that combined with Foreshortening distortion, the number of objects now in your focal range will change. If you view the 55mm image, you can see that the sprinkler the bush to the right of the sprinkler and the black tree trunk have all been pushed off into the distance this has now effectively moved them further away from the focal object (the rose plant in the green pot) So they do not exhibit the same level of focus that you see in the 300mm image. But as all images were shot at F-11 the depth of field is deep enough to cover most of the main objects in the frame. As all shots were taken at F-11 they all have about the same amount of Depth of Field because each shot was taken at about the same magnification.


iSSUE #2 TELECONVERTERS
Wow the results of this were considerably different than the above test. Here it's extremely evident that Teleconverters do impact on frame quality. Check out the green grass bush on the left of the frame, there is a clear lack of clarity and sharpness in the 600mm image compared to the 300mm image.

When comparing the 55mm image to the 300mm image, there is very little difference in object clarity and sharpness, so I would suggest where possible use a prime lens rather than adjusting focal length with teleconverters if you wish to retain maximum fine detail in your subject.


iSSUE #3 PERSPECTiVE and FORESHORTENiNG DiSTORTiON
As expected this is considerably evident in all images. At 55 meter object distance there's considerably more foreshortening distortion than at 5 meter object distance. Check out the black tree stump in the center of the frame AND the trees behind (slightly to the right) of this tree stump, in the 55mm image all are pushed off well into the distance, but in the 300mm image they appear considerably closer, and at 600mm look even closer and now totally fill the background behind our main object.. Even though the 55mm lens has pushed the background further away it does not mean these objects are in better or worse focus than at 300mm. Focal length changes the perspective between the objects, but it does not allow these objects to be in better focus.. They only appear closer to the focal point, but they are not in better (or worse) focus because of this. If you took the 55mm lens photo and enlarged the stump to the same size as seen in the 300mm or 600mm shot, it would look pretty much the same.


( Above images were scanned in using a Epson Perfection 2480 PHOTO flatbed scanner (scanning the 35mm negative) at 2400 x 4800 DPI, then precise bicubic resized down to 896 x 600 resolution to make viewing easier in a 1024 x 768 desktop (stored as JPEG). )




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Created on: 9th September 2oo4
Last Updated on: 16th November 2oo4

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