The need for "converting" focal lengths

Stratman · 08-07-2007, 06:35 PM

Since we're talking about FOV here, i have a question.. On my Kit Lens, which is designed for the digital sensor, at 18 mm, will it have the same FOV as an 18mm film lens on a film SLR?

PDL · 08-08-2007, 12:33 AM

If you have a 100mm lens that puts an object that is 15mm x 15mm on a given sensor - it does not matter what the so called crop factor is. If you have a piece of 35mm film the 100mm lens will make the object 15mm x 15mm on the film - same goes for the AP-C sensors we use in our Pentax DSLR's.

When looked at from the entire sensor - the 15mm x 15mm object will take up more area of a AP-C sensor than a 35mm frame. The focal length does not change - the size of the object projected onto the sensor does not change. All that has changed is the size of the sensor. Crop factor is a poor choice of words - but it provides a frame of reference back to the 70+ years of 35mm photography.

To Stratman - the field of view is smaller on digital AP-C sensors - because the sensor is smaller 17 x 23mm as opposed to 24 x 36mm for 35mm film. To understand this idea better - visit this site - this page Digital Camera Sensor Sizes: How it Influences Your Photography

PDL

stewart_photo · 08-08-2007, 12:38 AM

Originally posted by Stratman

Since we're talking about FOV here, i have a question.. On my Kit Lens, which is designed for the digital sensor, at 18 mm, will it have the same FOV as an 18mm film lens on a film SLR?

Digital lenses project a smaller image circle on the smaller sensor, allowing for less glass, lower weight, and smaller designs. One side benefit of a smaller lens design is often shorter internal movement typically resulting in somewhat quicker auto focusing. However, the crop factor or focal length multiplier (FLM) is the same for these lenses as any other. Therefore, assuming your camera has a FLM of 1.5, your digital lens with a physical focal length of 18mm would have a FOV similar to a 27mm lens on a 35mm film camera.

stewart

Stratman · 08-08-2007, 04:27 AM

Ok, this question has been in my head for a while, and now i have an answer !! Thanks all !! Now to decide what REALLY wide angle lens i want, hehe

chedoy · 08-09-2007, 09:48 PM

Stratman, one more thing to consider ... if that 18mm lens was designed for an APS digital sensor, it may not throw as much light onto the imaging surface if used on a film camera. Result is vignetting at the corners.

The concept of FOV doesn't really need to be understood if one sticks with one camera family with the same size sensor and only buys new lenses. It's something to keep in mind though, if you were looking to migrate older film lenses to a new digital body.

regken · 08-10-2007, 02:57 AM

I still have a need to convert mm to something more related to my experience. In my case that is what the eye perceives as natural. In the case of a 35mm camera that is about 50mm and a 1.5 crop sensor it is about 33mm. Being an old race course junkie I think in terms of what magnification you get out of a pair of 10X binoculars. The K10D would need a 330mm lens to equal that view and a 35mm camera would need a 500mm lens.

With that in mind all I want is a lens equal to a Bushnell 20X spotting scope at the same price, LOL.

Regards,

Ken

08-08-2007, 12:38 AM	#18
stewart_photo Veteran Member Join Date: Apr 2007 Location: Heidelberg, Germany Posts: 1,864	Originally posted by Stratman Since we're talking about FOV here, i have a question.. On my Kit Lens, which is designed for the digital sensor, at 18 mm, will it have the same FOV as an 18mm film lens on a film SLR? Digital lenses project a smaller image circle on the smaller sensor, allowing for less glass, lower weight, and smaller designs. One side benefit of a smaller lens design is often shorter internal movement typically resulting in somewhat quicker auto focusing. However, the crop factor or focal length multiplier (FLM) is the same for these lenses as any other. Therefore, assuming your camera has a FLM of 1.5, your digital lens with a physical focal length of 18mm would have a FOV similar to a 27mm lens on a 35mm film camera. stewart Last edited by stewart_photo; 08-08-2007 at 12:41 AM. Reason: Looks like we're writing things at almost the same time, PDL.

08-10-2007, 02:57 AM	#21
regken Veteran Member Join Date: Sep 2006 Location: Bangor, Maine Photos: Gallery Posts: 1,377	Still have a need to convert I still have a need to convert mm to something more related to my experience. In my case that is what the eye perceives as natural. In the case of a 35mm camera that is about 50mm and a 1.5 crop sensor it is about 33mm. Being an old race course junkie I think in terms of what magnification you get out of a pair of 10X binoculars. The K10D would need a 330mm lens to equal that view and a 35mm camera would need a 500mm lens. With that in mind all I want is a lens equal to a Bushnell 20X spotting scope at the same price, LOL. Regards, Ken

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08-07-2007, 06:35 PM	#16
Stratman Veteran Member Join Date: Mar 2007 Location: St Louis, Missouri U S A Photos: Gallery \| Albums Posts: 2,464	Since we're talking about FOV here, i have a question.. On my Kit Lens, which is designed for the digital sensor, at 18 mm, will it have the same FOV as an 18mm film lens on a film SLR?

08-08-2007, 12:33 AM	#17
PDL Pentaxian Join Date: Oct 2006 Location: PNW USA Photos: Gallery Posts: 2,128	If you have a 100mm lens that puts an object that is 15mm x 15mm on a given sensor - it does not matter what the so called crop factor is. If you have a piece of 35mm film the 100mm lens will make the object 15mm x 15mm on the film - same goes for the AP-C sensors we use in our Pentax DSLR's. When looked at from the entire sensor - the 15mm x 15mm object will take up more area of a AP-C sensor than a 35mm frame. The focal length does not change - the size of the object projected onto the sensor does not change. All that has changed is the size of the sensor. Crop factor is a poor choice of words - but it provides a frame of reference back to the 70+ years of 35mm photography. To Stratman - the field of view is smaller on digital AP-C sensors - because the sensor is smaller 17 x 23mm as opposed to 24 x 36mm for 35mm film. To understand this idea better - visit this site - this page Digital Camera Sensor Sizes: How it Influences Your Photography PDL

08-08-2007, 04:27 AM	#19
Stratman Veteran Member Join Date: Mar 2007 Location: St Louis, Missouri U S A Photos: Gallery \| Albums Posts: 2,464	Ok, this question has been in my head for a while, and now i have an answer !! Thanks all !! Now to decide what REALLY wide angle lens i want, hehe

08-09-2007, 09:48 PM	#20
chedoy Senior Member Join Date: Dec 2006 Location: Chicago, IL Posts: 143	Stratman, one more thing to consider ... if that 18mm lens was designed for an APS digital sensor, it may not throw as much light onto the imaging surface if used on a film camera. Result is vignetting at the corners. The concept of FOV doesn't really need to be understood if one sticks with one camera family with the same size sensor and only buys new lenses. It's something to keep in mind though, if you were looking to migrate older film lenses to a new digital body.