lenses designed for digital

Being a relative newcomer to photography, i still don't understand
why a lens designed for a digital camera needs to be quoted as having a crop factor compared to 35mm. Surely if it designed for the digital sized sensor no other dimension need to be taken into consideration.
confused am i

This 'crop factor' business, although doubt intended to be helpful, has turned out to cause nothing but confusion. A 50mm lens, say, remains a 50mm lens no matter what format, 35mm, 120 or whatever. The only thing that changes is the angle of view.

As more people like yourself take up photography with no experience of other formats then hopefully this silliness will die out. If I were you I would ignore it.

Lens sizes (50mm, 200mm etc) are still quoted in 35mm sizes even though they're'digital' lenses. this helps stop confusion when using older lenses designed for film as well as digital designs.

Think how confusing it would be if the DA* 200 was called DA*300, but only produced the same size image as a SMC 200.

That's how I see it, we just have to live with it.

On the plus side it's not just Pentax that do this everybody does.

Why on earth would you call a '200mm' lens a '300mm' lens when it patently is not.

Its simply a form of reference to help you visualise what the lens will cover, not much help if you've never used a 35mm camera though

35mm has been around for so long and become so popular that its kind of become the "standard" by which everything else is measured, look at it from this point of view; why is your car engines output measured in "horse power"

Alan, Even horse power is now no longer used in car engines, they refer now to 105ps or 77kW, all very confusing for someone as old as I am.

Thats exactly the point I was making, in an engines spec sheet the bhp is still included (bracketed) as a referance because many motorist will have no idea what "77kw" means in terms of performance

People still buy pounds of potatoes and say the temperature is 65Farenheit. They (and I) will say "Jeez, that's thirty bob!"

It's only a matter of familiarity.

Where lenses are concerned, if a given lens can be fitted to a range of mounts with a range of sensor sizes (only two, at present, I think) it's essential that the focal length of the lens is used to describe it. Any other description is open to misunderstanding. (Even quoting the 35mm equivalent can introduce problems because you can end up doing the conversion twice!)

How many serious users of a DSLR do the conversion from 200mm to 300mm when selecting which lens to use? They just get to associate a certain focal length with an appropriate range of subjects and distances. You get to know what to expect when you look through the viewfinder.

The "Equivalent Game" is just there for the benefit of lens manufacturers. They want to make out you're getting more for your money. They omit to point out that, for wide angle lenses, you are getting LESS.

The focal length of a lens does not change - a 50mm will be 50mm on an SLR and 50mm on a DSLR.

What everyone became used to was that 50mm was a "standard" which approximated to a typical human field of view of around 45 degrees (without moving your eyeballs). And from that you could work out the magnification of a telephoto - 200mm >> 4x, 300mm >> 6x, 400-600mm would zoom from 8x to 12x &c.

People now need to use a different base line - although with different sensor is can vary slightly, the factor being around 1.5, 1.55, 1.6.

With those you get a base line of 32 or 33 mm and everything should be taken from there. Where the figure settles? Who can predict. It could be 30, 32, 35 ... And then you would use that figure to look at the performance of other lenses.

Using 32 as the base, a 16mm will be 0.5x, 135mm >> ~4.2, 200mm >> 6.25 &c.

So, forget the "crop factor" and 50mm, put 32mm in your head as the standard and work from that.

On 35mm film typically an 85 mm lens would be used for portraits and that had a 1.7 magnification. Applying that to a DSLR to would get 1.7*32 = 54.4 which is rather close to the 55mm lens in the digital range.

Actually PS is the German abbreviation for Pferdestärke which means horse strength. There are minor differences (around 1%) between the imperial mechanical horsepower and the metric horsepower and the German. So, when you see PS just read it as HP and you will be pretty close.

I'll agree with that.

But I will add that a 'Standard' 18-55 digital lens can be used on some film bodies with no problems above 24mm (I think), I know, I've done it.

As for 200mm becoming 300mm, that's what the crop factor does to the lens. My AGFA Issolette has an85mm lens on a large format body. the lens is the 120 film equivalent of a 35mm film 50mm lens.

But it's still called 85mm!

As 'spider said, 35mm film frame size has become a standard and other formats measure against it.

Lenses are called "digital" for one of two reasons:

1. The image circle is smaller than that of a "full frame" lens to reduce size and weight. This will cause some vignetting when used on a film camera.

2. The coatings are designed with digital cameras in mind to minimise reflections from the sensor. The coatings are good with film as well, though.

It's not to do with the focal length, despite the fact that you need a different focal length for APSC digital than 35mm film to get the same view.

The idea of the "35mm equivalent" business is to have a consistent reference when comparing different formats. I find this genuinely useful for comparing compact camera lenses between different cameras because there are so many different sensor sizes in that world. It's unnecessary and confusing though once you're sticking with one particular format, eg. Pentax digital.

It was the large number of different sensor sizes that probably created the "need" for the 35mm-equivalent idea. It's fine when the format is a standard one - we know a 110 camera needs a 24mm standard lens and that a 6x6cm one has a 75 or 80mm lens and when those were around camera users were part of a more specialised hobby.

Now photography is so universal not everyone knows what a standard lens is, never mind a 35mm equivalent. Imagine the confusion to a beginner of a 35mm lens being described as having a 35mm-equivalent focal length of 52.5mm.....

I think the 35mm-equivalent idea is fading already and for DSLRs that's fair enough. Soon we will have more people knowing what an APS-C field of view is than a 35mm film camera view is.

The problem will eventually die out.

May I suggest, that if you can, try purchasing some Full Frame lenses. Who knows, but a little bird tells me that a FF Pentax might come out and if so, your FF lenses will work perfectly on it. Perhaps you may also want to use a 35mm film camera on occasion and thus the FF option.

You may be right Ben, in which case my FA lenses might find new found favour. Although I do rather like the Quick Shift facility on the DA lenses...