Monday, February 9, 2009

The DSLR Multiplier Myth Exposed

Last month, a friend asked me about purchasing a 300mm lens for wildlife photography with his Nikon D80, instead of a 500mm lens. He thought he could save some cash since the "300mm lens would give me the same magnification as a 450mm lens, wouldn't it?"

Before discussing the details, let me get the answer to his question out of the way, "It won't!"

What my friend was talking about is the "multiplier effect." Today's expensive professional level digital single lens reflex cameras (DSLR) have full sized sensors, meaning their sensors have the same size as a 35mm negative. Other professional DSLR's, as well as consumer oriented DSLR cameras have smaller sensors. Nikon calls their smaller sensors, DX sensors and their full size sensors, FX. When comparing lenses mounted on a DSLR with a DX sensor to a 35mm film camera, or a DSLR with an FX sensor we "multiply" the lens' focal length by 1.5, so a 300mm lens becomes a 450mm lens, or does it?

What's really happening is the field of view is being reduced or cropped. Cropping is the process of cutting part of an image out of a photograph to make the final photo. In the case of a Nikon D80 with a DX sensor, with a 300mm lens attached, the effective field of view is that of a 450mm lens. The photo is cropped, not magnified.

Monument Valley Lens View FX-DXOn the right is a view of Monument Valley through the lens of a DSLR. Inside the white rectangle is the image picked up by a full size, FX sensor. Inside the yellow rectangle is the image picked up by a smaller, DX sensor.

Note that the DX image is a cropped portion of the FX image. The field of view has been narrowed along both axises, horizontal and vertical. There is less of the scene in the DX image than the FX image.

Look at the two resultant photos on the right, below the lens view. Both are printed to the screen at the same size. The FX image is at the top, and the DX image is on the bottom.

Monument Valley FX and DX imagesIf you examine them carefully you will find that the DX image appears more "magnified" than the FX image. That's happened because to print both photos to the same physical size, the smaller DX image was enlarged, to increase its size to the FX's image size.

This is where I believe the misunderstanding comes from, when discussing the "multiplier effect." Personally I think we would all be served better by eschewing the "multiplier" terminology and changing it to the "crop effect" and the "crop factor." If we used these terms from the beginning, I don't think anyone would think they could get more magnification from a 300mm lens than from a 400mm lens.

Now that we understand the crop (multiplier) effect, I can hear you ask, "So does the crop factor affect travel photography?" Yes it does, according to the situation. It depends on the type of photograph you're taking while traveling.
  • If you're taking portraits, wildlife or macro photos then the crop factor won't be an issue.
  • If you're taking landscape photos, or architectural photos in a crowded city, or indoor photos, the crop factor will impact your travel photography.
Often some of the most beautiful landscape photography has utilized wide-angle lenses to capture the scenery in front of the camera. As shown above, a DX based DSLR camera reduces, or crops the field of view. While a 28mm lens on an FX DSLR or 35mm film camera is considered to be a wide angle lens, it isn't on a DX DSLR camera. On a DX camera, the 28mm lens has the equivalent field of view of a 42mm lens.

This means that a landscape photographer using a DX DSLR needs to use extremely wide angle lenses to capture the same scenic panoramas as FX cameras. I use a Sigma 10mm-20mm zoom lens for my wide angle landscape photographs. It has the equivalent angle of view to a 15mm-30mm zoom lens which is a wide angle zoom lens specification in anyone's book.

If you've researched typical DSLR DX sensor based camera kits, I'm sure you've noticed that most come with a lens such as an 18mm-55mm zoom, which can provide a good angle of view for landscapes, architectural, and indoor photos. Now you know why.


Anonymous said...

When you write that it is cropped, does that imply that the sharpness of the photo would be detrimentally affected by using the 300mm lens on a DX sensor (cropped to 450mm) when compared* to the 450mm lens on an FX sensor?

(*for areas of the photo that overlap)

Ned S. Levi said...

Great question.

Cropping itself doesn't degrade image quality. Image quality does degrade if you enlarge the cropped image to the size the overall image was before cropping took place.

In DX vs. FX when using a 300mm lens, the magnification that the lens imparts to the image is the same for both sensors. The difference is on the FX sensor you're getting a full size image, while in the DX you're getting a smaller image as the sensor is smaller. The field of view is smaller on the DX, and so you get the effective field of view of a 450mm for a Nikon DX camera.

Now if you take the smaller image and blow it up to the size of the larger image, the image will be degraded, comparatively. (This is the same as digital zooming. That's when an taken is cropped, then blown up to the size of the original image. It then looks like it's been taken with a longer lens because it looks magnified, but the thing is, it's not. It's just an enlargement.)

So, to directly answer your question, the sharpness of the photo taken is the same for both cameras (assuming equal quality). Degradation will occur, only if you then try to enlarge the DX image to the physical size of the FX image.

I hope I've been clear. Thanks for your question.

Anonymous said...

Thanks Ned! That explains it beautifully.

Anonymous said...

From what i understand, the "multipler effect" or cropping is really a digital zoom, and as your friend is using it for wildlife photography, he is justified... is he?

Ned S. Levi said...

Thanks for your comment.

In my opinion, the "multiplier effect" is not the same as digital zoom.

In digital zoom, the camera takes a portion of the image, then enlarges it back to the full size of the camera’s sensor, thus "magnifying" the subject of the image, then saves the "magnified" image.

A DSLR camera without a "full size" sensor does no such processing. The notion of such a camera causing a "multiplier effect" is a misnomer. The image on a small size sensor based DSLR is saved exactly as it falls on the sensor. The DSLR is not magnifying the image when it processes and saves it.

What is really happening is that because the sensor is smaller than a full size sensor, the field of view of the lens is reduced. Put a 100mm lens on a "DX" sensor based camera, and the field of view is reduced to that of a 150mm lens. The camera's sensor only sees a portion of the image a full size sensor based DSLR sees.

The saved image from the small sized sensor based camera has no digitally based magnification whatsover. The only actual magnification you get, if any, is optical, provided by the lens.

The only way you get digital magnification is if in post processing you enlarge the photo, for example, to the natural size it would be had the sensor been a full size sensor.

As to the question, is anyone justified in using the "multiplier effect" to capture wildlife photography in a "magnified size," of course not.

The "multiplier effect" is a myth, as I explain in the article.

The "multiplier effect" doesn't change a 300mm lens into a 450mm lens with regard to magnification. Despite the "effect" a 300mm lens is still a 300mm lens, and produces the same magnification on a smaller sensor based camera, that it produces on a full size sensor based camera.

I hope you now understand what the "multiplier effect" really is.

xpose said...

Very detailed and helpful guides for cropping and long exposures technique. You must be a very experience photographer. Thanks so much

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