I'm PERPLEXED! Focal Length and angle of view on Sony A57

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I'm PERPLEXED! Focal Length and angle of view on Sony A57

Postposted on Sat Feb 15, 2014 8:28 pm

I am beyond confused right now. I have a Sony A57 camera, and have the kit lens which is 18mm-55mm Focal Length. I also have a Minolta Maxxum 24mm f2.8 lens that was originally made for a film camera, but fits on my Sony Alpha 57.

Why is the angle of view greater with my kit lens at 18mm, than with the Minolta prime lens at 24mm? :-? :-? :-? :-? :-?
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Re: I'm PERPLEXED! Focal Length and angle of view on Sony A5

Postposted on Sat Feb 15, 2014 10:06 pm

The focal length is, for all intents and purposes, how zoomed into the frame you are -- the higher the 'mm', the more zoomed in you are, and the lower the 'mm', the more zoomed out you are. Angle of view is how much of the scene you can get in each frame.

Lower number focal lengths are more zoomed out, so you get a wider field of view and can fit more in each frame. Your 18mm lens is a little more zoomed out than the 24mm lens, so you can get more stuff in each frame.
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Re: I'm PERPLEXED! Focal Length and angle of view on Sony A5

Postposted on Sat Feb 15, 2014 10:41 pm

An 18mm focal length gives a wider field of view than a 24mm focal length does. The 55mm focal length gives a narrower field of view.

While the APS-C size sensor on your DSLR captures a smaller field of view than full-frame 35mm film would with the same lens, the light passing through the glass still behaves the same, based on the focal length.
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Re: I'm PERPLEXED! Focal Length and angle of view on Sony A5

Postposted on Sat Feb 15, 2014 11:26 pm

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Re: I'm PERPLEXED! Focal Length and angle of view on Sony A5

Postposted on Sun Feb 16, 2014 12:10 am

Great camera! You may also want to check out dyxum.com. The information above is right on. I had had a similar question regarding why millimeters were used as a unit of measure versus the angle of view. It goes back to the 1700's and there we are. The 24/2.8 is a great lens. With the fast aperture, its a great lens for low light. Also, it is a great lens for landscape as it is sharp from corner to corner at f/5.6.
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Re: I'm PERPLEXED! Focal Length and angle of view on Sony A5

Postposted on Sun Feb 16, 2014 5:49 am

a57 owner here. I fricking love that camera. So glad I chose it over a Nikon D5100, which I was also considering at the time. My office has a D5100 and it sucks compared to the a57 in a few ways: focusing, electronic viewfinder, actuation speed, and probably more ways that I am too lazy to think of; image quality is comparable but I'd give the a57 a slight edge at higher ISO's. I'm just saying this to let you know you have a great camera.

To explain the focal length issue: take a piece of paper and cut a 1" hole in it and hold it in front of your eye. Notice that the close you hold it to your eye the wider angle of view you have through the hole. Lenses work similarly to that. Shorter focal length (distance to eye) is wider field of view.
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Re: I'm PERPLEXED! Focal Length and angle of view on Sony A5

Postposted on Sun Feb 16, 2014 9:45 am

^^This is so simple and brilliant.
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Re: I'm PERPLEXED! Focal Length and angle of view on Sony A5

Postposted on Mon Feb 17, 2014 6:46 pm

Thanks for the help guys. I promise I am not as ignorant as I came across in my post with such a goofy question. For some reason I thought that the focal length for a lens varied with sensor size, when in fact the focal length for a lens doesn't change if you put it on a camera with a smaller or larger sensor. I thought that I would have to divide the focal length of my lens by 1.5 (the crop factor for sony aps-c), which gives 16mm. I noticed that my 18mm kits lens was more zoomed out than my 24mm prime and got confused! Now I can see that zooming my kit lens to 24mm gives the same angle of view as the 24mm Minolta!

I didn't realize this 24mm f2.8 was so great! Apparently people love this thing?! I bought the Minolta 50mm f1.8 prime lens on ebay, and this Minolta 24mm f2.8 came with it but didn't think it was anything special.

My favorite lens is the Minolta "beercan" 70-210mm f4 lens. I love that constant aperture all the way to 210! Macro shots are great too. Any of you guys have this lens? I recommend it for the Alpha mount owners out there. It goes for ~$200 on ebay.


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I've been hunting for a good lens for taking pictures of stars. It sounds like a 24mm f1.4 is a great way to go, but I need something cheap!
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Re: I'm PERPLEXED! Focal Length and angle of view on Sony A5

Postposted on Mon Feb 17, 2014 7:21 pm

Gilligan wrote:Thanks for the help guys. I promise I am not as ignorant as I came across in my post with such a goofy question. For some reason I thought that the focal length for a lens varied with sensor size, when in fact the focal length for a lens doesn't change if you put it on a camera with a smaller or larger sensor.

Correct.

The way I like to describe it fairly straightforward. Imagine a pinhole camera: you have a box, with a light-sensitive plate or film of some sort at one end, and a tiny hole in the centre at the other end. The focal length of the lens is the distance that pinhole has to be from the plate to form the same image as the lens. Note that it doesn't matter how big the plate is; even if the lens only forms an image circle that covers only a part of the plate, it still has a focal length that matches the distance the pinhole needs to be to form the same image within that particular area.

Conversely, the field of view depends upon the distance the pinhole lies from the plate, as well as the size of the plate - basic trigonometry. If you cut the size of the plate in half in both dimensions, the area it captures from a given lens will be halved in both dimensions, giving a similar effect as if the focal length had been doubled - but the lens is still the same focal length; only the field of view has altered.

So a 50mm lens is "normal" on a standard 35mm film camera; a moderate telephoto on APS-C digital sensors; a longer telephoto for a compact camera (which generally has a tiny sensor); and a wide angle lens for a medium or large format camera. In all cases, the lens is still a 50mm focal length lens; it's the image sensor size that varies.

The major reason all this matters is depth of field, which is (to a first approximation) a function of the absolute aperture size: a 50mm f/2.0 lens has the same absolute aperture size as a 100mm f/4.0 lens, making it easier for a large format camera to get a shallow depth of field than a 35mm or APS-C camera (handwave, handwave.) Whether this is a good thing or a bad thing depends on what you're trying to achieve.
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Re: I'm PERPLEXED! Focal Length and angle of view on Sony A5

Postposted on Mon Feb 17, 2014 7:38 pm

Thanks for the further clarification. What about focus, does it slightly alter the angle of view?
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Re: I'm PERPLEXED! Focal Length and angle of view on Sony A5

Postposted on Mon Feb 17, 2014 7:46 pm

Focusing away from infinity certainly seems to increase the angle of view... I just did it with my camera.
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Re: I'm PERPLEXED! Focal Length and angle of view on Sony A5

Postposted on Mon Feb 17, 2014 7:58 pm

On front focusing lenses (you can focus by turning the very end of the lens, or when you focus/autofocus, the end of the lens rotates, or if it telescopes slightly) then yes, your angle of view will change somewhat as a result of focusing.

You kind had the right idea regarding the sensor size and how it affects the (relative) angle of view. Except I think you divided when you should have multiplied. If you use, for example, and 18mm lens made for a full frame camera (If they aren't made for that you may get intense vignetting) and compared the angle of view with that lens on a full frame camera VS a crop camera with a 1.5x crop factor, the angle of view on the second camera would effectively be 27mm (18mm * 1.5). It is like taking the original image from the full frame camera and cropping the outer portions.

In the analogy of holding up a piece of paper with a hole up to your eye, this is the equivalent of changing the size of your eye.

edit: Although now that I think about it, the effects of that may not be completely intuitive. A smaller sensor can give an apparent advantage when it comes to zooming/long focal lengths. Larger sensors with proper lenses can provide wider angles of view without as much stretching of proportions on the outsides of the frame, and also be less affected by small imperfections of the lens and chromatic aberration. Smaller sensors do not perform as well in low light (when the quality of the sensors is otherwise the same)
Last edited by MJZ82 on Mon Feb 17, 2014 8:03 pm, edited 2 times in total.
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Re: I'm PERPLEXED! Focal Length and angle of view on Sony A5

Postposted on Mon Feb 17, 2014 8:00 pm

I have had several Beercans. Never pay $200 for it. When I get a good one, I sell it for $130 if it has the hood. $100 for decent ones or good ones without the hood. There can be some lens variation (remember, it is a near-30 year old lens), and some copies are better than others. The constant f/4 really is good. But, I have an 80-200/2.8 which is stellar, so I just don't have any use for a beercan anymore. A couple of more lenses to look out for:

The Minolta 50/1.7 is a great lens. Very very sharp, and very very cheap. Can be had for ~$60. Good that you got one. Its a perfect indoor and low light shooting lens.

The Beercan - great lens and highly useful focal range. Also, the bokeh on it is extremely good, making it great for portraits (outdoors unless in a good studio).

If you need more length, only get the Minolta 100-300 APO or APO D. They run for $100 and $200, respectively.

By far the nicest lens you can get for cheap is the original 35-105. Its the same era as the Beercan so it has an all-metal body. Its a stellar performer and runs between $80-100. I have one coming soon I bought as a back up, but I just scored a "Secret Handshake", the 28-135mm of the same era, so I won't need it. Let me know if you are interested and I'll see if its a good copy or not.

Another great lens for APS-C (cropped sensors) is the Tokina 11-16/2.8. Great wide angle. Another excellent Sony is the 16-105mm. Amazingly sharp. If you want to do video, though, get the 18-135mm. Its amazing for video.
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Re: I'm PERPLEXED! Focal Length and angle of view on Sony A5

Postposted on Mon Feb 17, 2014 8:19 pm

For an all-around wide aperture zoom, the Tamron model A16 17-50mm f/2.8 Di-II is a popular choice.
http://www.photozone.de/sony-alpha-aps- ... 50_28_sony
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Re: I'm PERPLEXED! Focal Length and angle of view on Sony A5

Postposted on Mon Feb 17, 2014 8:22 pm

JustAnEngineer wrote:For an all-around wide aperture zoom, the Tamron model A16 17-50mm f/2.8 Di-II is a popular choice.
http://www.photozone.de/sony-alpha-aps- ... 50_28_sony


I had one of these and it was great for indoor shooting. Very sharp and a good performer. But, think of it as a replacement for your kit zoom, which is adequate.

Another great buy is the Sony 35/1.8. Under $200 used and it gives you a wider view than the 50mm.
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