Bigger lens = more light in?

[Aleks]

Hello, I have a question I've been wondering for a long time. I want to get a fast lens for my D3200 due to I want to film videos at low light for a budget and I have concluded to the 35mm 1.8g nikon lens and here is my question.
Does the 55mm FX 1.8g lens let more light in than the 35mm DX 1.8g lens? From the physical looks it seems that the 55mm lens lets in more light, I mean yeah both are 1.8g lens but one is an FX lens thats why I'm asking.

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[skene]

The amount of light that is let in, would be based on your aperture. Any lens that has a wide aperture and shallow DOF (F1.0, F1.2, F1.4, F1.8) vs the latter smaller aperture will allow less light in (F3.5, F4, etc).

 

[pedroj]

I think F1.8 with either lens lets the same light in,I don't think the focal length will alter that....

 

[RON_RIP]

But what about this: a 50 1.8 d has a 52 mm front lens and 50 1.8 g has a 58mm front element. Does one of these let in more light at the same aperture than the other. It seems that more professional lenses have larger front elements than prosumer lenses.

 

[BackdoorArts]

I have a lens with an 82mm front element and the biggest aperture is f/5.6 and it has trouble focusing on some cameras in dark situations.

It's not about the front element, it's all about the amount of light loss between the front and back elements with the aperture wide open. Bigger elements can help retain light, but it's only part of the equation. F/1.8 = F/1.8, regardless of the front element size.

 

[pedroj]

I assume F1.8 is F1.8 regardless of size of the front...

 

[WayneF]

Hello, I have a question I've been wondering for a long time. I want to get a fast lens for my D3200 due to I want to film videos at low light for a budget and I have concluded to the 35mm 1.8g nikon lens and here is my question.
Does the 55mm FX 1.8g lens let more light in than the 35mm DX 1.8g lens? From the physical looks it seems that the 55mm lens lets in more light, I mean yeah both are 1.8g lens but one is an FX lens thats why I'm asking.

No.

fstop = focal length / effective aperture diameter. A 200 mm f/4 lens has a 4x larger aperture diameter than does a 50 mm f/4 lens, but (unadjusted) the focal length is a factor of exposure too (it is a principle of magnification). So, we use the fstop numbering system instead of the diameter. It is designed like this specifically so that a f/1.8 lens compares to another f/1.8 lens. All lenses of same fstop should see the same exposure. It is the single purpose of fstop.

The DSLR (with larger sensor) will have much less depth of field at f/1.8 than would a tiny video camera with a tiny sensor (like compact cameras). You might not like depth of field in videos shot at f/1.8 with the larger sensor. But.... the larger sensor can use much higher ISO (less noise) than the tiny sensor, so f/5 at higher ISO might do as well.

 

[FastGlass]

Having an 50mm fx lense will let in more light than a 50mm DX lense mounted on an FX body. Look at the rear element on both lenses and compare. Put either on a DX body and all is the same.

 

[WayneF]

? The FX lens should cover the area of the larger FX sensor (projects a larger diameter circle), but the exposure at same fstop number is the same exposure in either DX/FX case. Use the same lens on both bodies, and it is of course exactly the same lens either way.

 

[Steve B]

Having an 50mm fx lense will let in more light than a 50mm DX lense mounted on an FX body. Look at the rear element on both lenses and compare. Put either on a DX body and all is the same.

It's a matter of letting in the same amount of light per a given sensor area. A lens set at 1.8 should let in the same amount of light per square mm on the face of the sensor no matter what. An FX lens is of course going to let in more total light since it throws a larger image circle but that is meaningless when it comes to exposure.

For your D3200 the results, and exposure will be exactly the same. If you plan to go full frame in the future buy the FX lens. Otherwise the DX lens is going to work fine and it will save you some money.

 

[gqtuazon]

I assume F1.8 is F1.8 regardless of size of the front...

Yes it will be f1.8 but each lens when mounted on the same camera may provide a variance in Iso level using the same shutter and aperture settings.



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[WayneF]

Yes it will be f1.8 but each lens when mounted on the same camera may provide a variance in Iso level using the same shutter and aperture settings.

Modern lens coatings make it be not much issue today though, well solved today. In the early days (before WWII lets say), uncoated lenses were a big problem of light loss. In the simplest case, a glass-air surface reflects maybe 4% of the light, and only 96% is transmitted through. This loss occurred at EVERY glass-air surface in the lens. In a complex lens with many glass elements (many instances of such loss), overall transmission was relatively low (requiring more exposure). Today (modern lens coatings), our zooms might have 15 or more glass elements in them (30 surfaces), but this would have been impossible in the old uncoated days.

It was less problem for still photography, since our metering methods are so variable anyway, and we look at single pictures. The big problem was movie cameras, where (before zooms), different lenses were rotated into position, and these switches could vary the exposure a little, but visibly. So Hollywood demanded lenses calibrated in actual light transmission, called T-stops instead of f/stops.

But coating technology makes it be little problem today. And even if our most complex lenses do suffer a tiny bit, our reflective metering is so variable (much bigger problem), so we would never be able to assign blame.

Nevertheless, regardless of history, the concept of fstop numbering is that lenses using same fstop will see the same exposure. That is the single purpose of inventing this fstop ratio.

 

[aroy]

The amount of light transmitted in a lense is given by its "T"value. Aperture is a rough indicator, but ultimately it is the T value that matters. To get an idea of T values of various lenses, use DXO "compare lenses" facility.

For still photography it does not matter much as the meter evaluates the light entering through the lense and adjusts the exposure accordingly. With video, things are a bit different as it is not possible to keep adjusting exposure automatically between frames. In most cases the aperture is changed manually (or with an add on device).

It is important to match the transmission of different lenses to be used in a video shoot. If T values do not match the video brightness will vary across lenses. That is why lenses used for video are matched for consistent T values.

If you are serious about video, it is better to get lenses which have aperture ring. In fact a lot of people prefer the older manual focus lenses for video. Nikon "D" lenses do not, MF and "D" lenses do. As Nikon cameras do not change the aperture constantly in video, your only recourse to adjust for variable light is to adjust the aperture on the fly. That is why lense for video have stepless aperture ring.

 

[PaulWog]

It may have been stated earlier, but here's where I think your confusion may be at:

A larger sensor takes in a greater amount of light because it's larger. The larger your format, the larger your lenses get. So, a fast aperture lens for full frame might be a bit larger than a fast aperture lens for a crop sensor camera. Letting in more light doesn't inherently mean more detail, more dynamic range, etc, etc. Simplified, aperture is all you really need to look at for your practical applications: An f1.8 lens that operates on FX, when put on DX, is still an f1.8 lens. It lets the same amount of light in.

 

[WayneF]

It may have been stated earlier, but here's where I think your confusion may be at:

A larger sensor takes in a greater amount of light because it's larger. The larger your format, the larger your lenses get. So, a fast aperture lens for full frame might be a bit larger than a fast aperture lens for a crop sensor camera. Letting in more light doesn't inherently mean more detail, more dynamic range, etc, etc. Simplified, aperture is all you really need to look at for your practical applications: An f1.8 lens that operates on FX, when put on DX, is still an f1.8 lens. It lets the same amount of light in.

Exactly right, but simply to provide easy examples:

Larger sensors do "collect more light" in the same sense that it takes more paint to paint a barn than to paint one board. More paint to cover the larger area.

But the color is the same either way (which is exposure, illumination intensity per square area).

Same way with f/stop.

Comparing a 200mm lens with a 50 mm lens:

The 200 mm lens magnifies the image, so (compared to a 50mm lens), (and standing in the same place), if we could see the same field of view, the larger 200mm lens image necessarily covers 4x sensor dimensions, which is 16x area (as compared to the 50 mm lens image). This requires 16x more light to cover the larger field with the same previous intensity (same light per square area).

Or (what we actually do), to fit both fields of view onto the same sensor area, the 200 mm lens has to stand back 4x farther (to see same view). This concentrates the same subject scene into a 1/4 size area, which is also 1/16 intensity.

So, to achieve the same exposure (in either example), the 200mm lens aperture must necessarily be 4x larger diameter, which is then the same 16x more intensity (Pi r squared, area is exposure).

This is what people are arguing, but it has all already been solved many years ago. Solution is called f/stops.

We know to instead beneficially use the f/stop numbers to judge exposures...

fstop = focal length / effective lens aperture. This equalizes the lenses, so that any lens at f/4 exposes at f/4. This is the intended single purpose of the f/stop numbering system.

I say effective diameter, because this is the apparent diameter seen when looking through the magnification of the front lens elements (the apparent size the scene sees). The physical diameter will be a bit different.