Lens Geeks - Thoughts?

[Sime]

From the support hotline...

"I was wondering if you could help settle an argument between a co-worker and I. In discussing the best lens option for another friend, we were arguing between a 50mm/1.8 and a 35mm/1.8 and which would perform better in low light. He argued that since the maximum diameter of the 50mm's aperture was 27.7mm, that would be better in low light than the 35mm's 19.4mm.

I assume that at the same maximum 1.8 aperture, the lenses would perform similarly if all things were held consistent. If there were a difference in light levels, it would be due to the difference in field of vision of the two lenses.

Any science behind either of these two ideas?"

--Bill D

[peetyvulture]

Well apertures numbers are actually a sum. The f/1.8 is the focal length divided by 1.8. So this means that the ratio between the two lenses would perform equally in low light, unless it has something to do with field of view like you said.

Hope that helped

[jota2101]

Hmm i think that it depends in where you would like to shoot.

In tight places, 35mm would do better than 50mm.
Remember the cropping factor in DX format. a 50mm in a DX sensor would be 75mm and 35mm would be a 50mm, 52.5 to be more precise.
I read that a 35mm DX lens on an FX body, goes really good.

I own a 50mm nikkor lens, and works really good in low light condition. Make sure your camera works neat under low light condition by adjusting ISO.

then, what would you like to shoot? both lenses have great capabilities, they do really good in shooting portraits. Maybe a 35mm is better for landscapes though.

35mm lens covers more angle, is better fro group shots.
Both have the same maximum aperture, so they would perform the same.

Please, correct me if i am wrong.

cheers!

[ravncat]

Field of view comes into play and the total flux matters over the area visible to the lens. It really is convenient that the f number is a ratio of focal length and lens diameter...


The amount of light captured by a lens is proportional to the area of the aperture, equal to:

Area = pi (f /(2N))^2

Where f is focal length and N is the f-number.

the relative amount of focal-plane illuminance is independant of the focal length and only depends on the f number. So it`s less at a longer length, higher f number and bigger format. (If both lenses pass the same amount of light) In the end the amount of light collected varies with 1/(f/2). - In fact, if we look at the amount of light coming through, you will find not all f 2.8 lenses to be equal. Zooms often loose a lot of light to transmissivity. The Number of elements and design can also lose internal light. Compare a simple 180mm prime design at 2.8 to a 70-200 2.8 zoom at 180 at 2.8. You`ll probably find differences in the light coming through. T stops are still important. (but only if you are using an external meter - through the lens metering gets rid of this problem nicely)

Another issue that comes into play is that there's a difference between light collected and the useful light collected. low f number (bigger aperture) lenses may collect more light, but the lens aberrations are key to determining the quality of the column of light coming out the back (remember that we`re not dealing with a simple lens and need our light to hit our sensor as close to perpendicular as possible) So - these aberrations go up rapidly as the F number decreases (say as you go from f 8 to f5.6 to f 4 towards f0 ).

This means once you get to large apertures, the beam isn:t so great and even a point source will include rays that are headed away from the collimated ideal - and part of the reason why - some of that extra light from the larger aperture of the 50 1.8 is getting lost inside the lens. (though they`ve got all kinda of crazy designs to deal with it)

[BryanC]

Originally Posted by Sime

From the support hotline...

"I was wondering if you could help settle an argument between a co-worker and I. In discussing the best lens option for another friend, we were arguing between a 50mm/1.8 and a 35mm/1.8 and which would perform better in low light. He argued that since the maximum diameter of the 50mm's aperture was 27.7mm, that would be better in low light than the 35mm's 19.4mm.

I assume that at the same maximum 1.8 aperture, the lenses would perform similarly if all things were held consistent. If there were a difference in light levels, it would be due to the difference in field of vision of the two lenses.

Any science behind either of these two ideas?"

--Bill D

The quick answer is that, under identical conditions, light intensity is the same for the same aperture across all focal lengths.

This follows the Inverse Square Law which states that the intensity of the light is inversely proportional to the square of the distance it travels.

The formula: Intensity = 1/(Distance x Distance)

Intensity = The area of the circle of the aperture
Distance = Focal Length

A practical example of this law in practice is with Constant Aperture Zooms. These types of lenses maintain the same aperture throughout the focal range of the lens by increasing or decreasing the area of the aperture opening, giving you the same exposure or intensity of light through the whole focal range.

[SwissJon]

The F stop number is calculated so that, no matter what the focal length of your lens, the same amount of light hits your sensor. It's this way because that's what the f/stop is defined as being, light intensity, and if it changed with focal length, the F/Stop would be a meaningless measurement (Like an elastic tape measure)

[Jim Poor]

Originally Posted by peetyvulture

Well apertures numbers are actually a sum. The f/1.8 is the focal length divided by 1.8. So this means that the ratio between the two lenses would perform equally in low light, unless it has something to do with field of view like you said.

Hope that helped

It's not a sum, it's a quotient.

[sybren]

It all depends on how you define "performs better in low light". If you're talking about motion blur due to the longer shutter, I think you'll be better off with the 35mm. With the same aperture you'll have the same shutter speed, but due to the fact that the 35mm is "zoomed out" with respect to the 50mm, the blur caused by the motion will also be smaller, hence less visible.

[ryder]

I really think this misses the point. While theoretically these would be the same in low light, there are many factors that can impact the image in real life. Most importantly, the design of the lens, the number of elements, and the number and types of coatings on the lens could all impact the actual image. Different manufacturers would be designed different and even if the same manufacturer was compared you would need to stay in the same quality range. I know Canon so I would not want to compare an EF lens from the L series to an EF-S consumer lens as it would be an unfair match. There is some truth to the old saying "you get what you pay for."

[ryder]

Canon does not make a 35mm f/1.8 lens but I would not expect the EF 50mm f/1.4 to compare favorably with the 35mm f/1.4 L series lens at three times the price. Canon likely has put much more effort and science into the higher priced lens, so in this case the 35mm should win hands down.

However you could compare the 35mm f/1.4 L series to the 50mm f/1.2 L series which are both similarly priced and the 50mm should win as it has the lower aperture number hence yielding more light. However the difference from f/1.2 to f/1.4 should not be a huge difference and I would say that the choice should be on angle of view, not f/ stops at this range.