Prime objective

[BlueFightingCat]

Hi,

I am considering getting a "prime" objective to use indoors, in low light conditions. Also it would be great for the added depth of field.

However I have a question. I have a Canon 450d DSLR which means its not full frame. Essentially I have to multiply the focus length by 1.6X. So I was wondering should I get a prime objective that is 50mm (meaning effectively 80mm) or a 30mm (which would give me 48mm effective).

BFC

[acedrew]

Depending on what you want to shoot, my 50mm is great for portraits, due to the 80mm equivelant, but it is a bit long to have on the camera all the time, I leave it on there anyway

Andrew Rodgers
Perfected Perspectives -- Photography by Andrew Rodgers | Andrew Rodgers (acedrew) on Twitter | Login | Facebook

[Shokinen]

I bought a 35mm because my feet can bring me closer of my subject if needed. The opposite may not be possible especially inside.

[inkista]

I use 35/50/85/135. They're all good. It depends on your working distance and subject matter and framing. If you have a zoom lens, try fixing it at 35 and then at 50 and seeing which one you prefer. Or use ExposurePlot to see which focal length you've used more.

The thing that's special about a 50mm, though, that no other lens can replicate, is that if you have it on your camera, and you look through the viewfinder with one eye, when you open the other eye the views will match. That's why 50mm is the "normal" focal length.

[Shokinen]

Originally Posted by inkista

The thing that's special about a 50mm, though, that no other lens can replicate, is that if you have it on your camera, and you look through the viewfinder with one eye, when you open the other eye the views will match. That's why 50mm is the "normal" focal length.

Even with a APS-C sensor ?

[inkista]

Originally Posted by Shokinen

Even with a APS-C sensor ?

Yup. Try it and see. Sensor size has no effect on this one. This is one of those object lessons on lenses folks need to learn: while the sensor size does affect the field of view, the magnification and perspective of the lens itself do not change just because it's mounted on another body; the focal length doesn't change--it's a physical property of the lens. "Focal length magnifier" is a misnomer.

When you look through the viewfinder with your 35mm lens mounted on the camera, you have roughly the same field of view as a 50mm on a full-frame, but if you look through the viewfinder and open your other eye, what you see through the viewfinder will be wider/smaller than with the unaided eye.

[Mr Guy]

Originally Posted by Shokinen

Even with a APS-C sensor ?

That's one of those yes with an if, and no with a but type answers. The whole normal == eye in the viewfinder is both mostly true but also mostly misleading. 50mm is generally considered normal because of the lack of perspective distortion on 36mm film. It'll look mostly 1:1 through most viewfinders, regardless of sensor, so it's kinda true still. However, the resultant images won't have that advantage on a crop body sensor, so it's not really a normal lens for those systems any more.

Remember though, the reason 50mm are considered normal isn't the same as the reason they are considered standard, although most people confuse that and infuse them with mystical properties of perspective. In actuality, they were only close to normal, for film, and the result was a convenient size and simple optical formula to mass produce cheaply.

[Shokinen]

Alright, let's put it with real example in order to understand this correctly.

If I put a 35mm on my D90, this should equal to 52.5mm on a FF.
If I put a 50mm on my D90, this should equal to 75mm on a FF.

Which lens is going to replace what I see in my viewfinder if I open the other one ?

(sorry I don't have a 50mm to test...)

[vandergus]

I think the problem is that we're mixing together viewfinder magnification and field of view. Viewfinder magnification has to do with how big something appears in the finder compared to how big it appears with the naked eye. Most viewfinders specify their magnification assuming a 50mm lens is attached because that was long considered the standard (most common, whatever) lens. For objects in the viewfinder to appear the same size as they do with the naked eye, you would need %100 magnification in the viewfinder. But even full frame cameras are only speced around 70-75% magnification with a 50mm lens so I'd be hesitant to say there's anything special about that particular focal length. The only thing really special about it is that Oscar Barnack happened to have one laying around his work shop.

With APS-C cameras, the viewfinder is basically cropped to match the size of the sensor, so you get the same magnification in the viewfinder but a smaller field of view. If you want a normal field of view (like a 35mm lens) then you get much less magnification in the viewfinder. This was one of the big complaints about these cameras when they were first introduced because everyone was used to the big bright finders in 35mm film cameras.

So magnification is what lets you open your other eye and see things the same size as you do in the viewfinder, and that's related to the lens' focal length and the camera's viewfinder. Field of view is related to the lens' focal length and the format of the sensor/film.

[ddr]

I guess you learn something new everyday=)

50mm is quite an odd focal length to use on a aps-c camera. My research has led me to an affordable sigma 50mm f/1.4. Any good suggestions for a 35mm lens other than the Canon L one?