Magnification and crop factor

[StanChung]

Zoom macros are mostly pseudo macro. 1:3 also can call themselves macro! Ruined the term 'macro' IMO.

FWIW-to add to Choy's explanation, 1:1 means the image printed onto the film is the same size as your subject at minimum focus. 1cm fish appears 1cm on the negative of a film camera.
1:2 means 1cm fish appears 0.5cm... 1:4 means 0.25cm Follow?

For digital like your D80, there's a 50% enlargement of the image size. eg a 90mm lens with a film camera is effectively a 135mm on D80.

As opposed to full frame digital camera's like Canon's EOS 5D.

Since you have insects[venomous?] in mind, probably good idea to have some working distance so that it doesn't take a hostile interest in you.
Longer focal length also means less DOF[depth of field]. So you need more light to work at smaller apertures to get more DOF. No problems with flash in this respect. SU800+ sync cord or just a white card bounce will do depending on how professional/impactful you want your images to look like.
3D moulding [back- highlight, side + fill...] or just whack from the front like family gathering pictures.

[hwchoy]

As opposed to full frame digital camera's like Canon's EOS 5D.

arggghhhhhhh!!!!!

do you HAVE TO MENTION that one and make me sleepless tonight!?

Glamour mah! I also have fantasy of having this lens to get only the eyes sharp. Nose, mouth all outfo [jargon for 'out of focus'].

For purist-pay more for lens and no need to learn photoshop and can act as smug proud owner of glamour lens with stunning bokeh[soft quality defocussed bg or fg ].

For advertising- to get good bokeh can photoshop with lens defocus to a certain extent . Not as good as the real thing though but more control if the client says bg too distracting...bla bla bla.

the great thing about large aperture (that means small ƒ number) lenses is that the barrels are THICK. girls get really impressed when they see you aim your lens with a BIG BIG hole in it and they will automatically pose and smile in your direction.

remember, from the girl's perspective, they cannot see how long your barrel is, but a BIG hole will really catch their attention

[valice]

For digital like your D80, there's a 50% enlargement of the image size. eg a 90mm lens with a film camera is effectively a 135mm on D80.

This don't sound really right. I think it is not an really an enlargement. But the amount of zoom is of 90mm but the crop is closer due to the reduced sensor size. So 1cm fish is still a 1cm fish on the smaller sensor just that you have reduced amount of "sides" around the subject.

arggghhhhhhh!!!!!

do you HAVE TO MENTION that one and make me sleepless tonight!?

Price dropped already leh.
BBB!!!!

[hwchoy]

This don't sound really right. I think it is not an really an enlargement. But the amount of zoom is of 90mm but the crop is closer due to the reduced sensor size. So 1cm fish is still a 1cm fish on the smaller sensor just that you have reduced amount of "sides" around the subject.

technically you are right, and that's why we say nikons have 1.5× crop factor. but the effect is SIMILAR to a 135mm lens when a 90mm is used on such bodies.

HOWEVER, the effect is not EXACTLY the same, as there are differences in the perspective (field of view) presented by lenses of physically different focal length.

[StanChung]

Lol. Choy, Sleep tight...

Hi Vincent,
Well from my 'understanding' , if you're standing in the same spot, whatever lens you are using is the same perspective but different crop/distortion depending on the lens generally speaking.

So in essence even if it's a 'crop', it still gives a larger image than a full frame digital camera of the same megapixel range, [not necessarily better quality pixels] if we're comparing apples with larger apples that is.

[if nobody understood what I said, it ok, it was late at night ]

If minimum focus being the same, wouldn't the image appear bigger theoretically?

[hwchoy]

see, I am sleepless, ok?


actually the focal length of the lens determines its perspective, or field of view. wide angle lens has a wide FOV (duh, that's why they call them wide angle, hee hee) and long tele lenses have very narrow FOV.

so standing at the same spot, you will have different perspective depending on the focal length of the lens you are looking through. and this is an optical property of the lens, unrelated to the crop factor of the sensor. Hence a 50mm on a full frame camera does not quite produce the same view as a 75mm on a 1.5× body.

[StanChung]

Yes, me too. for different reasons!

If standing on same spot, FOV is indeed determined by lens. Perspective however doesn't change.
Let me explain, erm, imagine lets say a full frame fish eye lens picture [16mm] of a field with forest and mountains, draw a smaller rectangle on it, that will give you a smaller field of view of perhaps a [24mm]. and so on and so forth. The smaller rectangles represent a subsequently longer lens but the perspective doesn't change. You cannot see what's behind the tree, whatever lens you're using because you're in the same spot!

The perspective you're talking about are lens effect perspective.
So perhaps mine is a different perspective on the term perspective. [pun intended ]

Still awake?

[valice]

Let me explain, erm, imagine lets say a full frame fish eye lens picture [16mm] of a field with forest and mountains, draw a smaller rectangle on it, that will give you a smaller field of view of perhaps a [24mm]. and so on and so forth. The smaller rectangles represent a subsequently longer lens but the perspective doesn't change. You cannot see what's behind the tree, whatever lens you're using because you're in the same spot!

I think if using this arguement and then you "enlarge" the "24mm" picture on a same piece of paper or screen as the "16mm", you sort of get a zoomed effect. But then the bokeh and DOF is still that of the 16mm and not 24mm...

[hwchoy]

this is not right. the reason is the "rectangles" you are trying to draw lies on an imaginary curved surface with you as the centre. what you describe "drawing rectangles on the pictures" is not accurate as the "sphere" has been flattened onto the paper or sensor already.

this effect is most apparently with wide lens.

[StanChung]

er...the rectangle must be drawn in the middle of the imaginary fish eye picture. Sorry, missed that...[late mah ]

Vincent, the bokeh is a lens characteristic, due to smaller/larger aperture, groups of elements, focussed point etc agree?

[valice]

Hmmm... can ignore the bokeh point. But DOF effect will be different.

[StanChung]

DOF will be different, because of the construction of the lenses, expecially the diameter, number of pieces of glass/distortion/compression. Can't go further already as this is the limit of my understanding.

[hwchoy]

DOF is affected by the following:

• focal length
• aperture
• subject distance
• size of sensor or print

essentially driven by the concept of "Circle of Confusion" which is the smallest discrete point/circle that can be resolved by the viewer.

[valice]

Choy, why does sensor size matter? I would think that it is due only to the first three points.

[hwchoy]

Choy, why does sensor size matter? I would think that it is due only to the first three points.

yes this is a bit counter-intuitive but going back to the concept of COC (Circle of Confusion).

First is to realise that there is only ONE infinitely thin plane which is where everything is in absolute sharp exact focus.

Every other plane in front and behind that is out of focus, to some degree.

Now at what point do you start to perceive this "slightly OOF" planes as APPEARING to be OOF? So now do you understand that OOF is not an absolute condition, and that it is not black and white whether a plane is OOF or not. It depends on how it is viewed, ie the size at which the image is being viewed.

e.g. if you take an image and print at ¼-A4, everything might look sharp. If you print it at A3, then you start to perceive that some parts of the image or less sharp than others.

so going back to a digicam sensor, which is a fraction the size of a 35mm film or DSLR sensor, the image that is form on this small sensor, would necessarily be very small also, and hence more of the image would "look" sharp to the sensor.

hence a digicam has much much more DOF because of a combination of small sensor and very short focal lengths (typically ƒ=5mm vs ƒ=50mm in DSLR).

[bclee]

Here is a good article on DOF:

http://www.cambridgeincolour.com/tut...h-of-field.htm

Here is a more detailed DOF calculator that takse print size, view distance into account.

http://www.cambridgeincolour.com/tut...calculator.htm

BC