Manual Lensometer question..

[Berno]

Hi everyone!

I have a question about the difference between a manual Lensometer.. which has what I believe is called a corona-style target (a cross with a circle of dots in the middle) versus the cross-line target (which is a cross of 3 seperated lines and 2 thinner lines)
I've never worked with latter and was wondering if the measuring system is different..

With the first one (the cross and circle of dots in the middle) if for example [working with - cyl ]:

The higher plus clears one line at +1.75 @ 35
Then I keep going toward the minus and get the other line clear at +1.00 (@125) The Rx will be:
+1.75 -1.00 X 125

Someone told me that wth the cross-line target (assuming the same scenario above) the axis will be x35 and I should get the sphere lines (the thin ones) clear first..

I'd really appreciate it if you could clarify that for me.. I always thought there's only one principle of measuring and never knew it depends on the target etc.. should it be really different?

Thanks for any info...!
Ben:cheers:

[For-Life]

well there is no real "this is the way you do it." I find it best to get the sphere, and then find the cyl.

Does that help?

[harry a saake]

easist way, take a pair of glasses that you already know is ground to a specific rx and axis, measure it on the new lensometer, setting the axis wheel on the correct axis of course. then read the prescription and which ever set of dots comes out to the sphere power, thats the one. personally i have used this instrument and i dont have much use for it as i dont see the purpose, when they just as well could have put lines and accomplished the same purpose.

[chip anderson]

With any manual lensometer find the power and axis of the stongest power first. That being easier to read accurately, then find the cylinder.

Chip

[mullo]

Does your corona target lensometer have an axis wheel? If it does, then it should have lines and a corona target in the middle of the sphere/cylinder lines which is a cross-lined instrument.

If not.........with your corona target instrument your corona lines would become clear @ +1.75 for your sphere power and point to axis 035°. Then when you turn your power drum further in the minus direction you will see the corona lines become clear at @ +1.00 and point to the axis of your Rx = 125°. Your Rx would then be +1.75 -0.75 x 125.

For the cross-line target you are referring to you would see the following. Your sphere lines which could be found in any instrument by placing your power drum at "plano" and axis wheel at 180°......you will see your sphere lines sitting along the 090° vertical axis in the instrument.

Once you have identified which are your sphere lines then you would see them clear at power drum reading +1.75 and axis wheel reading of 125° but they will be sitting along the 035° axis when you look into the instrument. Turning your power wheel in the minus direction you will then see your cylinder lines clear at power drum reading of +1.00 but the cylinder lines will sit along the 125° meridian in the instrument.

Your Rx would be +1.75 -0.75 x 125.

I hope this helps.

[Barry Santini]

for very weak cyls, try *blurring* the target...you'll be surprised how much easier it is to determine the precise axis, if that is your goal.

FWIW

Barry

[QDO1]

Hi everyone!

I have a question about the difference between a manual Lensometer.. which has what I believe is called a corona-style target (a cross with a circle of dots in the middle) versus the cross-line target (which is a cross of 3 seperated lines and 2 thinner lines)
I've never worked with latter and was wondering if the measuring system is different..

With the first one (the cross and circle of dots in the middle) if for example [working with - cyl ]:

The higher plus clears one line at +1.75 @ 35
Then I keep going toward the minus and get the other line clear at +1.00 (@125) The Rx will be:
+1.75 -1.00 X 125

Someone told me that wth the cross-line target (assuming the same scenario above) the axis will be x35 and I should get the sphere lines (the thin ones) clear first..

I'd really appreciate it if you could clarify that for me.. I always thought there's only one principle of measuring and never knew it depends on the target etc.. should it be really different?

Thanks for any info...!
Ben:cheers:

Makes absoloutley no odds

If working in +cyl, find the least positive meridian first (align axis wheel), then the most positive. The line you are looking at will be alligned allong the axis

If working in -Cyl, find the least negative meridian first, (align axis wheel), then the most negative. the line youare looking at will be alligned allong the axis

I often dont even look at the numbers that much
its the way you think it

.. example i think.. right "thats in focus... i have -1.00. (keep moving focus dial in negative direction), and another 3 small divisions (=another -0.75) at what ever axis it is at

giving -1.00 - 0.75 at whatever


it's training your hand and your eye - my hand moves the dial this way = more negative, my hand moves the dial the other way = more positive

I move the lens and get with movement = positive
i move the lens and get against movement = negative
etc

With regards to the targets, I prefer the hybrid you get on the older topcon lensmeters - a circle of dots with lines that start slightly outside this

- on some cyls the dots are invaluable
- on aligning lenses prior to lens dotting up - the lines are invaluable. the thinner the lines the more accurate it all seems

the lines are also useful for getting a more accurate cyl measure (by lookingthrough the eye piece) as once the spectacles are removed.. you have a nice line pointing right at the peripheral axis scale allong one of the cross cyls

I would hate to work with just the circular target, as essentailly, centering is more of a judgement than a "that is bang along the line" process

Also: You can see lens distortions, and power fall-offs more easily with the lines. And in this respect every trick you have in the bag is a good one

[Berno]

Thank you all for your replies this is all make sense and helped a lot!!

Thank you ALL!:cheers:

[Luis davila]

Hi, Its very simple is the first thing you have to say.
well first you have to find find the two lines starting by the high positive power ,that will be the spherical power. Example. +3.50 Then look for the three lines that will be the the other power. The difference between them will be the cil power. You have to take a look in the last reading because this will be the axis of the lens.

Ex: first reading: +3.50 mires two lines
second reading +2.00 90° mires three lines ( here you see the axis)
So you take the first one as the spherical and the difference the cil. If you do this the cil allways be negative.

RX: +3-50 - 1.50 * 90